Being a Tolkien Elf elsewhere I use names and places that Word doesn't like all
the time - as for Sindarin (principal elf language of ME)
Because of that I don't use Word too much now for prepping forum stuff except for long lore or fanfic posts. I have now given up doing full doc spell or grammar checks altoghether as they're far too vague or far too pedantic in illogical bursts and now just pick up whether the wiggly red or green lines are correct in telling me something's incorrect as I go - the red wiggly lines I do take more notice of, although I have my own internal UK English filter (my brain of course) switched to 'ignore once/all' almost constantly (as my default language selector is determined to revert to US English whenever possible) so I've never got to the point where I've filled my custom dictionary....
A pox on Microsoft I say!