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.