[Bugfix][Router] Default transcription language to None (auto-detect)

src/tests/test_multipart_proxy.py

@@ -165,7 +165,8 @@ async def transcribe(request):
         received.update(
             model=form["model"],
             response_format=form["response_format"],
-            language=form["language"],
+            # absent by default so the backend auto-detects; None documents that
+            language=form.get("language"),
             filename=upload.filename,
             content_type=upload.content_type,
             content=upload.file.read(),
@@ -186,7 +187,7 @@ async def transcribe(request):
     assert received == {
         "model": AUDIO_MODEL,
         "response_format": "text",
-        "language": "en",
+        "language": None,
         "filename": "speech.wav",
         "content_type": "audio/wav",
         "content": b"fake-audio",

src/vllm_router/services/request_service/request.py

@@ -1161,7 +1161,7 @@ async def route_general_transcriptions(
         temperature: Optional[float] = (
             float(temperature_str) if temperature_str is not None else None
         )
-        language: Optional[str] = form.get("language", "en")
+        language: Optional[str] = form.get("language")
         stream: bool = form.get("stream", "false").lower() == "true"
     except KeyError as e:
         return JSONResponse(
@@ -1191,7 +1191,16 @@ async def route_general_transcriptions(
     payload_bytes = await file.read()
     files = {"file": (file.filename, payload_bytes, file.content_type)}
 
-    data = {"model": model, "language": language}
+    data = {"model": model}
+
+    if isinstance(language, str):
+        language_stripped = language.strip()
+        if language_stripped and language_stripped.lower() not in (
+            "none",
+            "null",
+            "undefined",
+        ):
+            data["language"] = language_stripped
 
     if prompt:
         data["prompt"] = prompt