I'm coming from Windows and Linux world, so don't be too harsh :-)
Is it possible to develop a spelling dictionary and grammar support for unsupported languages?
Is there any documentation available? After some searching, I didn't find anything useful.
On the topic of spelling, you could start further research at https://www.cs.hmc.edu/~geoff/ispell.html. That will provide you a list of dictionaries for already-supported languages, and a framework for developing your own. I'd suggest that ispell is fairly well regarded, but is not the only spelling system for macOS/Unix.
As for grammar, a whole other kettle of fish that is.
MacOS can normally use any dictionary file available for OpenOffice and its relatives. Here is one source. These come from Hunspell.
