as a workaround for me it worked to word with the web version of Teams and use Teams in the web browser via office.com
change the browser language to your designated language and activate spell checking, then it works
unfortunately that still does not work, when using two languages all the time. Or even three. Then half the conversations I would have in browser, half in the app, very unconvinient.
As we have more than one tennant, we use that feature for haveing two active tennants at once, but for chats, this is too much of an inconvinience for me. I disabled the spellcheck completely, as the red underlining annoys me. Same as here, browser is set to English, I write in English, but spellcheck seems to take the system language (that I cannot change).
To be honest, I simply do not understand, how that feature has been removed. It was there and working, and somehow someone or something took it away. Initially I thought that the feature is added to some premium license model, but no, it is just gone.