Hi,
From following thread, it turned that if one language has been added and installed basic typing, it should be returned in the supported language.
https://stackoverflow.com/questions/55927304/add-spell-check-languages-for-the-windows-built-in-spell-check-provider
I did some research as well and found there is no documentation confirming the supported languages of Microsoft spelling check API but only one sentence mentioned that:
For languages that are not supported by the Windows Spellcheck API, Microsoft Edge will fall back to Hunspell.
https://www.thurrott.com/cloud/web-browsers/235809/microsoft-enables-windows-spellcheck-api-for-all-chromium-browsers
Please note: Information posted in the given link is hosted by a third party. Microsoft does not guarantee the accuracy and effectiveness of information.
Hope this helps and please help to accept as Answer if the response is useful.
Thanks,
Jenny