Fix bug in 8896899216

Closes #2215

yt_dlp/postprocessor/ffmpeg.py

@@ -775,7 +775,7 @@ class FFmpegMetadataPP(FFmpegPostProcessor):
         stream_idx = 0
         for fmt in info.get('requested_formats') or []:
             stream_count = 2 if 'none' not in (fmt.get('vcodec'), fmt.get('acodec')) else 1
-            lang = ISO639Utils.short2long(fmt['language']) or fmt.get('language')
+            lang = ISO639Utils.short2long(fmt.get('language') or '') or fmt.get('language')
             for i in range(stream_idx, stream_idx + stream_count):
                 if lang:
                     metadata[str(i)].setdefault('language', lang)