diff --git a/yt_dlp/extractor/common.py b/yt_dlp/extractor/common.py
index 795105b7d8..ecece85f5f 100644
--- a/yt_dlp/extractor/common.py
+++ b/yt_dlp/extractor/common.py
@@ -47,6 +47,7 @@
     FormatSorter,
     GeoRestrictedError,
     GeoUtils,
+    ISO639Utils,
     LenientJSONDecoder,
     Popen,
     RegexNotFoundError,
@@ -3071,7 +3072,11 @@ def _parse_ism_formats_and_subtitles(self, ism_doc, ism_url, ism_id=None):
             url_pattern = stream.attrib['Url']
             stream_timescale = int_or_none(stream.get('TimeScale')) or timescale
             stream_name = stream.get('Name')
-            stream_language = stream.get('Language', 'und')
+            # IsmFD expects ISO 639 Set 2 language codes (3-character length)
+            # See: https://github.com/yt-dlp/yt-dlp/issues/11356
+            stream_language = stream.get('Language') or 'und'
+            if len(stream_language) != 3:
+                stream_language = ISO639Utils.short2long(stream_language) or 'und'
             for track in stream.findall('QualityLevel'):
                 KNOWN_TAGS = {'255': 'AACL', '65534': 'EC-3'}
                 fourcc = track.get('FourCC') or KNOWN_TAGS.get(track.get('AudioTag'))