diff --git a/src/pipecat/services/asyncai/tts.py b/src/pipecat/services/asyncai/tts.py
index 0f72b3b9c..1923bb035 100644
--- a/src/pipecat/services/asyncai/tts.py
+++ b/src/pipecat/services/asyncai/tts.py
@@ -42,14 +42,17 @@ except ModuleNotFoundError as e:
     raise Exception(f"Missing module: {e}")
 
 
-def language_to_async_language(language: Language) -> str | None:
+def language_to_async_language(language: Language) -> str:
     """Convert a Language enum to Async language code.
 
     Args:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding Async language code, or None if not supported.
+        The corresponding service language code. If ``language`` is not in
+        the verified mapping, falls back to the base language code (e.g.,
+        ``en`` from ``en-US``) and logs a warning (via
+        ``resolve_language(..., use_base_code=True)``).
     """
     LANGUAGE_MAP = {
         Language.EN: "en",
diff --git a/src/pipecat/services/aws/tts.py b/src/pipecat/services/aws/tts.py
index d49aa796a..a683a94ea 100644
--- a/src/pipecat/services/aws/tts.py
+++ b/src/pipecat/services/aws/tts.py
@@ -37,14 +37,16 @@ except ModuleNotFoundError as e:
     raise Exception(f"Missing module: {e}")
 
 
-def language_to_aws_language(language: Language) -> str | None:
+def language_to_aws_language(language: Language) -> str:
     """Convert a Language enum to AWS Polly language code.
 
     Args:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding AWS Polly language code, or None if not supported.
+        The corresponding service language code. If ``language`` is not in
+        the verified mapping, falls back to the full language code string and
+        logs a warning (via ``resolve_language(..., use_base_code=False)``).
     """
     LANGUAGE_MAP = {
         # Arabic
diff --git a/src/pipecat/services/azure/common.py b/src/pipecat/services/azure/common.py
index 8bb48cd04..c2f21f1b1 100644
--- a/src/pipecat/services/azure/common.py
+++ b/src/pipecat/services/azure/common.py
@@ -9,14 +9,16 @@
 from pipecat.transcriptions.language import Language, resolve_language
 
 
-def language_to_azure_language(language: Language) -> str | None:
+def language_to_azure_language(language: Language) -> str:
     """Convert a Language enum to Azure language code.
 
     Args:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding Azure language code, or None if not supported.
+        The corresponding Azure language code. If ``language`` is not in the
+        verified mapping, falls back to the full language code string and logs
+        a warning (via ``resolve_language(..., use_base_code=False)``).
     """
     LANGUAGE_MAP = {
         # Afrikaans
diff --git a/src/pipecat/services/azure/stt.py b/src/pipecat/services/azure/stt.py
index 72a8f76b0..e130fe980 100644
--- a/src/pipecat/services/azure/stt.py
+++ b/src/pipecat/services/azure/stt.py
@@ -182,9 +182,9 @@ class AzureSTTService(STTService):
         changed = await super()._update_settings(delta)
 
         if "language" in changed:
-            self._speech_config.speech_recognition_language = (
-                self._settings.language or language_to_azure_language(Language.EN_US)
-            )
+            self._speech_config.speech_recognition_language = assert_given(
+                self._settings.language
+            ) or language_to_azure_language(Language.EN_US)
             if self._audio_stream:
                 await self._disconnect()
                 await self._connect()
diff --git a/src/pipecat/services/camb/tts.py b/src/pipecat/services/camb/tts.py
index 7b3aab768..2449a13dd 100644
--- a/src/pipecat/services/camb/tts.py
+++ b/src/pipecat/services/camb/tts.py
@@ -44,14 +44,17 @@ MODEL_SAMPLE_RATES: dict[str, int] = {
 }
 
 
-def language_to_camb_language(language: Language) -> str | None:
+def language_to_camb_language(language: Language) -> str:
     """Convert a Pipecat Language enum to Camb.ai language code.
 
     Args:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding Camb.ai language code (BCP-47 format), or None if not supported.
+        The corresponding Camb.ai language code (BCP-47 format). If
+        ``language`` is not in the verified mapping, falls back to the base
+        language code (e.g., ``en`` from ``en-US``) and logs a warning (via
+        ``resolve_language(..., use_base_code=True)``).
     """
     LANGUAGE_MAP = {
         Language.EN: "en-us",
diff --git a/src/pipecat/services/cartesia/tts.py b/src/pipecat/services/cartesia/tts.py
index 6b3fc4b83..d88f462c5 100644
--- a/src/pipecat/services/cartesia/tts.py
+++ b/src/pipecat/services/cartesia/tts.py
@@ -62,14 +62,17 @@ class GenerationConfig(BaseModel):
     emotion: str | None = None
 
 
-def language_to_cartesia_language(language: Language) -> str | None:
+def language_to_cartesia_language(language: Language) -> str:
     """Convert a Language enum to Cartesia language code.
 
     Args:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding Cartesia language code, or None if not supported.
+        The corresponding service language code. If ``language`` is not in
+        the verified mapping, falls back to the base language code (e.g.,
+        ``en`` from ``en-US``) and logs a warning (via
+        ``resolve_language(..., use_base_code=True)``).
     """
     LANGUAGE_MAP = {
         Language.AR: "ar",
diff --git a/src/pipecat/services/deepgram/flux/base.py b/src/pipecat/services/deepgram/flux/base.py
index 1d8ad4794..f2042d799 100644
--- a/src/pipecat/services/deepgram/flux/base.py
+++ b/src/pipecat/services/deepgram/flux/base.py
@@ -32,7 +32,7 @@ from pipecat.utils.time import time_now_iso8601
 from pipecat.utils.tracing.service_decorators import traced_stt
 
 
-def language_to_deepgram_flux_language(language: Language) -> str | None:
+def language_to_deepgram_flux_language(language: Language) -> str:
     """Convert a Pipecat Language to a Deepgram Flux language code.
 
     Only honored by the ``flux-general-multi`` model. Locale variants
diff --git a/src/pipecat/services/elevenlabs/stt.py b/src/pipecat/services/elevenlabs/stt.py
index 5571f8700..6fed6c5e5 100644
--- a/src/pipecat/services/elevenlabs/stt.py
+++ b/src/pipecat/services/elevenlabs/stt.py
@@ -54,7 +54,7 @@ except ModuleNotFoundError as e:
     raise Exception(f"Missing module: {e}")
 
 
-def language_to_elevenlabs_language(language: Language) -> str | None:
+def language_to_elevenlabs_language(language: Language) -> str:
     """Convert a Language enum to ElevenLabs language code.
 
     Source:
@@ -64,7 +64,9 @@ def language_to_elevenlabs_language(language: Language) -> str | None:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding ElevenLabs language code, or None if not supported.
+        The corresponding service language code. If ``language`` is not in
+        the verified mapping, falls back to the full language code string and
+        logs a warning (via ``resolve_language(..., use_base_code=False)``).
     """
     LANGUAGE_MAP = {
         Language.AF: "afr",  # Afrikaans
diff --git a/src/pipecat/services/elevenlabs/tts.py b/src/pipecat/services/elevenlabs/tts.py
index 4da94643c..239e6a64e 100644
--- a/src/pipecat/services/elevenlabs/tts.py
+++ b/src/pipecat/services/elevenlabs/tts.py
@@ -69,14 +69,17 @@ ELEVENLABS_MULTILINGUAL_MODELS = {
 }
 
 
-def language_to_elevenlabs_language(language: Language) -> str | None:
+def language_to_elevenlabs_language(language: Language) -> str:
     """Convert a Language enum to ElevenLabs language code.
 
     Args:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding ElevenLabs language code, or None if not supported.
+        The corresponding service language code. If ``language`` is not in
+        the verified mapping, falls back to the base language code (e.g.,
+        ``en`` from ``en-US``) and logs a warning (via
+        ``resolve_language(..., use_base_code=True)``).
     """
     LANGUAGE_MAP = {
         Language.AR: "ar",
diff --git a/src/pipecat/services/fal/stt.py b/src/pipecat/services/fal/stt.py
index 1e18c7f84..5aa16c09c 100644
--- a/src/pipecat/services/fal/stt.py
+++ b/src/pipecat/services/fal/stt.py
@@ -28,14 +28,17 @@ from pipecat.utils.time import time_now_iso8601
 from pipecat.utils.tracing.service_decorators import traced_stt
 
 
-def language_to_fal_language(language: Language) -> str | None:
+def language_to_fal_language(language: Language) -> str:
     """Convert a Language enum to Fal's Wizper language code.
 
     Args:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding Fal Wizper language code, or None if not supported.
+        The corresponding service language code. If ``language`` is not in
+        the verified mapping, falls back to the base language code (e.g.,
+        ``en`` from ``en-US``) and logs a warning (via
+        ``resolve_language(..., use_base_code=True)``).
     """
     LANGUAGE_MAP = {
         Language.AF: "af",
diff --git a/src/pipecat/services/gladia/stt.py b/src/pipecat/services/gladia/stt.py
index ec3277943..cc8e5279b 100644
--- a/src/pipecat/services/gladia/stt.py
+++ b/src/pipecat/services/gladia/stt.py
@@ -56,14 +56,17 @@ except ModuleNotFoundError as e:
     raise Exception(f"Missing module: {e}")
 
 
-def language_to_gladia_language(language: Language) -> str | None:
+def language_to_gladia_language(language: Language) -> str:
     """Convert a Language enum to Gladia's language code format.
 
     Args:
         language: The Language enum value to convert.
 
     Returns:
-        The Gladia language code string or None if not supported.
+        The corresponding Gladia language code. If ``language`` is not in
+        the verified mapping, falls back to the base language code (e.g.,
+        ``en`` from ``en-US``) and logs a warning (via
+        ``resolve_language(..., use_base_code=True)``).
     """
     LANGUAGE_MAP = {
         Language.AF: "af",
@@ -361,7 +364,7 @@ class GladiaSTTService(WebsocketSTTService):
             language: The Language enum value to convert.
 
         Returns:
-            The Gladia language code string or None if not supported.
+            The Gladia language code string, or None if not supported.
         """
         return language_to_gladia_language(language)
 
diff --git a/src/pipecat/services/google/gemini_live/llm.py b/src/pipecat/services/google/gemini_live/llm.py
index d6966cbc1..adcbe28b8 100644
--- a/src/pipecat/services/google/gemini_live/llm.py
+++ b/src/pipecat/services/google/gemini_live/llm.py
@@ -111,7 +111,7 @@ MAX_CONSECUTIVE_FAILURES = 3
 CONNECTION_ESTABLISHED_THRESHOLD = 10.0  # seconds
 
 
-def language_to_gemini_language(language: Language) -> str | None:
+def language_to_gemini_language(language: Language) -> str:
     """Maps a Language enum value to a Gemini Live supported language code.
 
     Source:
@@ -121,7 +121,9 @@ def language_to_gemini_language(language: Language) -> str | None:
         language: The language enum value to convert.
 
     Returns:
-        The Gemini language code string, or None if the language is not supported.
+        The Gemini language code string. If ``language`` is not in the
+        verified mapping, falls back to the full language code string and logs
+        a warning (via ``resolve_language(..., use_base_code=False)``).
     """
     LANGUAGE_MAP = {
         # Arabic
diff --git a/src/pipecat/services/google/stt.py b/src/pipecat/services/google/stt.py
index d9fb0a160..a386566c5 100644
--- a/src/pipecat/services/google/stt.py
+++ b/src/pipecat/services/google/stt.py
@@ -60,14 +60,17 @@ except ModuleNotFoundError as e:
     raise Exception(f"Missing module: {e}")
 
 
-def language_to_google_stt_language(language: Language) -> str | None:
+def language_to_google_stt_language(language: Language) -> str:
     """Maps Language enum to Google Speech-to-Text V2 language codes.
 
     Args:
         language: Language enum value.
 
     Returns:
-        Optional[str]: Google STT language code or None if not supported.
+        The corresponding Google STT language code. If ``language`` is not
+        in the verified mapping, falls back to the full language code string
+        and logs a warning (via
+        ``resolve_language(..., use_base_code=False)``).
     """
     LANGUAGE_MAP = {
         # Afrikaans
diff --git a/src/pipecat/services/google/tts.py b/src/pipecat/services/google/tts.py
index b507fc26f..58bc31691 100644
--- a/src/pipecat/services/google/tts.py
+++ b/src/pipecat/services/google/tts.py
@@ -60,7 +60,7 @@ except ModuleNotFoundError as e:
     raise Exception(f"Missing module: {e}")
 
 
-def language_to_google_tts_language(language: Language) -> str | None:
+def language_to_google_tts_language(language: Language) -> str:
     """Convert a Language enum to Google TTS language code.
 
     Source:
@@ -70,7 +70,9 @@ def language_to_google_tts_language(language: Language) -> str | None:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding Google TTS language code, or None if not supported.
+        The corresponding service language code. If ``language`` is not in
+        the verified mapping, falls back to the full language code string and
+        logs a warning (via ``resolve_language(..., use_base_code=False)``).
     """
     LANGUAGE_MAP = {
         # Arabic
@@ -219,7 +221,7 @@ def language_to_google_tts_language(language: Language) -> str | None:
     return resolve_language(language, LANGUAGE_MAP, use_base_code=False)
 
 
-def language_to_gemini_tts_language(language: Language) -> str | None:
+def language_to_gemini_tts_language(language: Language) -> str:
     """Convert a Language enum to Gemini TTS language code.
 
     Source:
@@ -229,7 +231,9 @@ def language_to_gemini_tts_language(language: Language) -> str | None:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding Gemini TTS language code, or None if not supported.
+        The corresponding service language code. If ``language`` is not in
+        the verified mapping, falls back to the full language code string and
+        logs a warning (via ``resolve_language(..., use_base_code=False)``).
     """
     LANGUAGE_MAP = {
         # Afrikaans (Preview)
diff --git a/src/pipecat/services/gradium/stt.py b/src/pipecat/services/gradium/stt.py
index 6f1a48f60..e58383b45 100644
--- a/src/pipecat/services/gradium/stt.py
+++ b/src/pipecat/services/gradium/stt.py
@@ -79,14 +79,17 @@ def _input_format_from_encoding(encoding: str, sample_rate: int) -> str:
     return encoding
 
 
-def language_to_gradium_language(language: Language) -> str | None:
+def language_to_gradium_language(language: Language) -> str:
     """Convert a Language enum to Gradium's language code format.
 
     Args:
         language: The Language enum value to convert.
 
     Returns:
-        The Gradium language code string or None if not supported.
+        The corresponding Gradium language code. If ``language`` is not in
+        the verified mapping, falls back to the base language code (e.g.,
+        ``en`` from ``en-US``) and logs a warning (via
+        ``resolve_language(..., use_base_code=True)``).
     """
     LANGUAGE_MAP = {
         Language.DE: "de",
diff --git a/src/pipecat/services/lmnt/tts.py b/src/pipecat/services/lmnt/tts.py
index 81dffac7e..7972780fc 100644
--- a/src/pipecat/services/lmnt/tts.py
+++ b/src/pipecat/services/lmnt/tts.py
@@ -37,14 +37,17 @@ except ModuleNotFoundError as e:
     raise Exception(f"Missing module: {e}")
 
 
-def language_to_lmnt_language(language: Language) -> str | None:
+def language_to_lmnt_language(language: Language) -> str:
     """Convert a Language enum to LMNT language code.
 
     Args:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding LMNT language code, or None if not supported.
+        The corresponding service language code. If ``language`` is not in
+        the verified mapping, falls back to the base language code (e.g.,
+        ``en`` from ``en-US``) and logs a warning (via
+        ``resolve_language(..., use_base_code=True)``).
     """
     LANGUAGE_MAP = {
         Language.AR: "ar",
diff --git a/src/pipecat/services/minimax/tts.py b/src/pipecat/services/minimax/tts.py
index 25b7feade..3ef1b49c3 100644
--- a/src/pipecat/services/minimax/tts.py
+++ b/src/pipecat/services/minimax/tts.py
@@ -31,14 +31,16 @@ from pipecat.transcriptions.language import Language, resolve_language
 from pipecat.utils.tracing.service_decorators import traced_tts
 
 
-def language_to_minimax_language(language: Language) -> str | None:
+def language_to_minimax_language(language: Language) -> str:
     """Convert a Language enum to MiniMax language format.
 
     Args:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding MiniMax language name, or None if not supported.
+        The corresponding MiniMax language name. If ``language`` is not in
+        the verified mapping, falls back to the full language code string and
+        logs a warning (via ``resolve_language(..., use_base_code=False)``).
     """
     LANGUAGE_MAP = {
         Language.AF: "Afrikaans",
diff --git a/src/pipecat/services/neuphonic/tts.py b/src/pipecat/services/neuphonic/tts.py
index a7303da6b..6a405a1d2 100644
--- a/src/pipecat/services/neuphonic/tts.py
+++ b/src/pipecat/services/neuphonic/tts.py
@@ -44,14 +44,17 @@ except ModuleNotFoundError as e:
     raise Exception(f"Missing module: {e}")
 
 
-def language_to_neuphonic_lang_code(language: Language) -> str | None:
+def language_to_neuphonic_lang_code(language: Language) -> str:
     """Convert a Language enum to Neuphonic language code.
 
     Args:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding Neuphonic language code, or None if not supported.
+        The corresponding service language code. If ``language`` is not in
+        the verified mapping, falls back to the base language code (e.g.,
+        ``en`` from ``en-US``) and logs a warning (via
+        ``resolve_language(..., use_base_code=True)``).
     """
     LANGUAGE_MAP = {
         Language.DE: "de",
diff --git a/src/pipecat/services/nvidia/stt.py b/src/pipecat/services/nvidia/stt.py
index 49fb4573e..d7064c04c 100644
--- a/src/pipecat/services/nvidia/stt.py
+++ b/src/pipecat/services/nvidia/stt.py
@@ -49,7 +49,7 @@ except ModuleNotFoundError as e:
     raise Exception(f"Missing module: {e}")
 
 
-def language_to_nvidia_nemotron_speech_language(language: Language) -> str | None:
+def language_to_nvidia_nemotron_speech_language(language: Language) -> str:
     """Maps Language enum to NVIDIA Nemotron Speech ASR language codes.
 
     Source:
@@ -59,7 +59,9 @@ def language_to_nvidia_nemotron_speech_language(language: Language) -> str | Non
         language: Language enum value.
 
     Returns:
-        str | None: NVIDIA Nemotron Speech language code or None if not supported.
+        The NVIDIA Nemotron Speech language code. If ``language`` is not in
+        the verified mapping, falls back to the full language code string and
+        logs a warning (via ``resolve_language(..., use_base_code=False)``).
     """
     LANGUAGE_MAP = {
         # Arabic
diff --git a/src/pipecat/services/sarvam/tts.py b/src/pipecat/services/sarvam/tts.py
index 1ab69ef15..100df80af 100644
--- a/src/pipecat/services/sarvam/tts.py
+++ b/src/pipecat/services/sarvam/tts.py
@@ -216,14 +216,16 @@ def get_speakers_for_model(model: str) -> list[str]:
     return list(TTS_MODEL_CONFIGS["bulbul:v2"].speakers)
 
 
-def language_to_sarvam_language(language: Language) -> str | None:
+def language_to_sarvam_language(language: Language) -> str:
     """Convert Pipecat Language enum to Sarvam AI language codes.
 
     Args:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding Sarvam AI language code, or None if not supported.
+        The corresponding service language code. If ``language`` is not in
+        the verified mapping, falls back to the full language code string and
+        logs a warning (via ``resolve_language(..., use_base_code=False)``).
     """
     LANGUAGE_MAP = {
         Language.BN: "bn-IN",  # Bengali
diff --git a/src/pipecat/services/smallest/tts.py b/src/pipecat/services/smallest/tts.py
index 6a2071f44..80f98cb54 100644
--- a/src/pipecat/services/smallest/tts.py
+++ b/src/pipecat/services/smallest/tts.py
@@ -51,14 +51,17 @@ class SmallestTTSModel(StrEnum):
     LIGHTNING_V3_1 = "lightning-v3.1"
 
 
-def language_to_smallest_tts_language(language: Language) -> str | None:
+def language_to_smallest_tts_language(language: Language) -> str:
     """Convert a Language enum to a Smallest TTS language string.
 
     Args:
         language: The Language enum value to convert.
 
     Returns:
-        The Smallest language code string, or None if unsupported.
+        The corresponding Smallest language code. If ``language`` is not in
+        the verified mapping, falls back to the base language code (e.g.,
+        ``en`` from ``en-US``) and logs a warning (via
+        ``resolve_language(..., use_base_code=True)``).
     """
     LANGUAGE_MAP = {
         Language.AR: "ar",
diff --git a/src/pipecat/services/stt_service.py b/src/pipecat/services/stt_service.py
index 7c6028ea1..e54581919 100644
--- a/src/pipecat/services/stt_service.py
+++ b/src/pipecat/services/stt_service.py
@@ -269,7 +269,10 @@ class STTService(AIService):
             language: The language to convert.
 
         Returns:
-            The service-specific language identifier, or None if not supported.
+            The service-specific language identifier. Return ``None`` to
+            indicate an unsupported language. This optional return is an
+            extension hook for future or third-party subclasses; as of
+            2026-04-28, first-party services return a string.
         """
         return Language(language)
 
diff --git a/src/pipecat/services/tts_service.py b/src/pipecat/services/tts_service.py
index 46c5c7528..f386b5bcb 100644
--- a/src/pipecat/services/tts_service.py
+++ b/src/pipecat/services/tts_service.py
@@ -467,7 +467,10 @@ class TTSService(AIService):
             language: The language to convert.
 
         Returns:
-            The service-specific language identifier, or None if not supported.
+            The service-specific language identifier. Return ``None`` to
+            indicate an unsupported language. This optional return is an
+            extension hook for future or third-party subclasses; as of
+            2026-04-28, first-party services return a string.
         """
         return Language(language)
 
diff --git a/src/pipecat/services/whisper/base_stt.py b/src/pipecat/services/whisper/base_stt.py
index 9ac84c41c..6a6bd6b12 100644
--- a/src/pipecat/services/whisper/base_stt.py
+++ b/src/pipecat/services/whisper/base_stt.py
@@ -40,7 +40,7 @@ class BaseWhisperSTTSettings(STTSettings):
     temperature: float | None | _NotGiven = field(default_factory=lambda: NOT_GIVEN)
 
 
-def language_to_whisper_language(language: Language) -> str | None:
+def language_to_whisper_language(language: Language) -> str:
     """Maps pipecat Language enum to Whisper API language codes.
 
     Language support for Whisper API.
@@ -50,7 +50,10 @@ def language_to_whisper_language(language: Language) -> str | None:
         language: A Language enum value representing the input language.
 
     Returns:
-        str or None: The corresponding Whisper language code, or None if not supported.
+        The corresponding service language code. If ``language`` is not in
+        the verified mapping, falls back to the base language code (e.g.,
+        ``en`` from ``en-US``) and logs a warning (via
+        ``resolve_language(..., use_base_code=True)``).
     """
     LANGUAGE_MAP = {
         Language.AF: "af",
@@ -235,7 +238,7 @@ class BaseWhisperSTTService(SegmentedSTTService):
             language: The Language enum value to convert.
 
         Returns:
-            str or None: The corresponding service language code, or None if not supported.
+            The corresponding service language code, or None if not supported.
         """
         return language_to_whisper_language(language)
 
diff --git a/src/pipecat/services/whisper/stt.py b/src/pipecat/services/whisper/stt.py
index 47f896056..443664348 100644
--- a/src/pipecat/services/whisper/stt.py
+++ b/src/pipecat/services/whisper/stt.py
@@ -97,14 +97,17 @@ class MLXModel(Enum):
     LARGE_V3_TURBO_Q4 = "mlx-community/whisper-large-v3-turbo-q4"
 
 
-def language_to_whisper_language(language: Language) -> str | None:
+def language_to_whisper_language(language: Language) -> str:
     """Maps pipecat Language enum to Whisper language codes.
 
     Args:
         language: A Language enum value representing the input language.
 
     Returns:
-        str or None: The corresponding Whisper language code, or None if not supported.
+        The corresponding service language code. If ``language`` is not in
+        the verified mapping, falls back to the base language code (e.g.,
+        ``en`` from ``en-US``) and logs a warning (via
+        ``resolve_language(..., use_base_code=True)``).
 
     Note:
         Only includes languages officially supported by Whisper.
@@ -300,7 +303,7 @@ class WhisperSTTService(SegmentedSTTService):
             language: The Language enum value to convert.
 
         Returns:
-            str or None: The corresponding Whisper language code, or None if not supported.
+            The corresponding Whisper language code, or None if not supported.
         """
         return language_to_whisper_language(language)
 
diff --git a/src/pipecat/services/xai/stt.py b/src/pipecat/services/xai/stt.py
index 3cc3a23b2..e219cfb6e 100644
--- a/src/pipecat/services/xai/stt.py
+++ b/src/pipecat/services/xai/stt.py
@@ -44,7 +44,7 @@ except ModuleNotFoundError as e:
     raise Exception(f"Missing module: {e}")
 
 
-def language_to_xai_stt_language(language: Language) -> str | None:
+def language_to_xai_stt_language(language: Language) -> str:
     """Convert a Language enum to the xAI STT language code.
 
     xAI STT accepts two-letter language codes (e.g. ``en``, ``fr``, ``de``,
@@ -54,7 +54,10 @@ def language_to_xai_stt_language(language: Language) -> str | None:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding xAI STT language code, or None if not supported.
+        The corresponding service language code. If ``language`` is not in
+        the verified mapping, falls back to the base language code (e.g.,
+        ``en`` from ``en-US``) and logs a warning (via
+        ``resolve_language(..., use_base_code=True)``).
     """
     LANGUAGE_MAP = {
         Language.AR: "ar",
diff --git a/src/pipecat/services/xai/tts.py b/src/pipecat/services/xai/tts.py
index 83e7781f2..ad7f05bbd 100644
--- a/src/pipecat/services/xai/tts.py
+++ b/src/pipecat/services/xai/tts.py
@@ -49,14 +49,17 @@ except ModuleNotFoundError as e:
     raise Exception(f"Missing module: {e}")
 
 
-def language_to_xai_language(language: Language) -> str | None:
+def language_to_xai_language(language: Language) -> str:
     """Convert a Language enum to xAI language code.
 
     Args:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding xAI language code, or None if not supported.
+        The corresponding service language code. If ``language`` is not in
+        the verified mapping, falls back to the base language code (e.g.,
+        ``en`` from ``en-US``) and logs a warning (via
+        ``resolve_language(..., use_base_code=True)``).
     """
     LANGUAGE_MAP = {
         Language.AR: "ar-EG",
diff --git a/src/pipecat/services/xtts/tts.py b/src/pipecat/services/xtts/tts.py
index f9caa3819..d323e9b67 100644
--- a/src/pipecat/services/xtts/tts.py
+++ b/src/pipecat/services/xtts/tts.py
@@ -37,14 +37,17 @@ from pipecat.utils.tracing.service_decorators import traced_tts
 # https://github.com/coqui-ai/xtts-streaming-server
 
 
-def language_to_xtts_language(language: Language) -> str | None:
+def language_to_xtts_language(language: Language) -> str:
     """Convert a Language enum to XTTS language code.
 
     Args:
         language: The Language enum value to convert.
 
     Returns:
-        The corresponding XTTS language code, or None if not supported.
+        The corresponding service language code. If ``language`` is not in
+        the verified mapping, falls back to the base language code (e.g.,
+        ``en`` from ``en-US``) and logs a warning (via
+        ``resolve_language(..., use_base_code=True)``).
     """
     LANGUAGE_MAP = {
         Language.CS: "cs",