UN M.49 Geographic Regions not supported
If you go to https://unstats.un.org/unsd/methodology/m49/ and click Geographic Regions on the left sidebar, there are a few more codes that are not supported by MKVToolNix v50. Out of these, es-419 is commonly used for Spanish (Latin America).
❯ mkvmerge -o out.mkv --language 0:es-419 in.mkv
mkvmerge v50.0.0.1 ('Awakenings') 64-bit
Error: 'es-419' is not a valid IETF BCP 47/RFC 5646 language tag in '--language 0:es-419'. Additional information from the parser: The value '419' is not a valid UN M.49 country number code.