For me there is a workaround.
I have Evernote on iPhone, and iPad.
I compose the note with reminder and tags on the iPad, there the spell check/prediction works.
Then I save it and it shows perfectly on the desktop app.
I'll be the first to admit that it's a far from ideal solution.
Also, to design a successor for the perfectly reliable older edition, which clearly does not work with Windows 10 Language settings., is nothing less than shoddy workmanship.
I hope this helps FBZ