Jellyfin uses only ISO 639-2 language codes
to translate the metadata tag eng
to English
for the track selector.
For some languages multiple such tags exist, for example Germany has ger
and deu
.
Here only the tag deu
is correctly translated to German
whilst ger
remains unrecognized.
mkvmerge
always replaces the language codes with the matroska specified set of langauge codes,
which sadly are the wrong ones for Jellyfin.
As an end user one can use ffmpeg to rewrite the metadata language tags to the correct set for Jellyfin.