I have prepared a NSSpellServer spelling and grammar checker for Slovenian proofing in macOS. My proofing service gets used when I explicitly set keyboard spelling language to "Slovenian (Besana)" (my proofing tool).
When I set keyboard language to Automatic by Language, system does the language detection. Since it has limited support for Slovenian language, it doesn't recognize the text as Slovenian and never asks my proofing service to check the spelling. It does consult my proofing service for spelling suggestions and when it does, I see the language parameter there is set to anything but Slovenian.
Is it possible to install own language detector to macOS? Or, place some language dictionary files somewhere, the system could use to extend language detection to new languages?