Had a the same problem too
Open word, bottom left is your input language, mine was Irish(Ireland)
got to Review, set language, and you'll see you default language highlighted in blue, if your language doesn't have ABC with a check mark under it (scroll down the list of languages and you'll see others with it) then your spell check won't work.
Excel's work's because the dictionary was set by default to English(UK)
You can either do the same in word or check the region and language setting in control panel and make sure that isn't overriding your word language.
I'm Irish so You'll have figure out which languages you should be using.
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