This is extremely frustrating. The spell check works for some words but not for really basic words. For example, I wrote "Tehy require..." and even in this site, it autocorrected it to "They" but had to misspell it again for it to stick. Even this site, which I am using MS Edge for, right-click and knows it's "They" but Teams shows "Tethys" as the recommended change. This happens for many, many words that are only off by a letter or letters switched. My language in Teams is set to English US.
There is also no "Add to Dictionary" so can't get it to learn basic words.
@Mercedes Green I'm taking this issue up as my own personal crusade for Microsoft Teams. Please see the similar suggestion "Add to Dictionary" (the only similar suggestion which has received any attention from a Microsoft employee) and help by adding your voice to the discussion. This is a disgraceful omission from this product and must be corrected as soon as possible.