I'm not sure if this is a LibreElec issue or it is an underlying problem coming from Kodi 19.4 since its release notes mentions "Fix: Wrong language identified from subtitle name corrected".
After upgrading my RPi4 from LE 10.0.1 to LE 10.0.2 a subtitle "movie.pt-BR.srt" is identified as "Breton - pt" instead of "Portuguese - br". With LE 10.0.1 the language is correctly identified.