I suppose you’re requesting for Plex to support ISO-639-3 language codes.
From the documentation / support articles, Plex appears to support ISO-639-1 (2-letter) and ISO-639-2/B (3-letter) codes. Mayan and Jamaican languages are not part of that spec.
Though there’s already a dedicated suggestion to support Mayan language (even if the suggestion currently has no votes).
From https://support.plex.tv/articles/200220677-local-media-assets-movies/
Where
[Language_Code]is defined by the ISO-639-1 (2-letter) or ISO-639-2/B (3-letter) standard.Related Page: ISO-639-1 codes (2-letter)
Related Page: ISO-639-2/B codes (3-letter)
