Currently, Microsoft Teams does not provide a direct option for users to change the spell-checking language within a chat. The spell-checking language is typically determined by the language settings of the Teams client and the installed language packs. If you have installed multiple language packs, Teams should ideally support spell-checking in those languages, but it may not allow you to manually select the language for spell-checking in chat messages.
If Teams is highlighting your text in red, it may be due to the default language setting not matching the language you are using. You may want to check your Teams language settings and ensure that they align with the language you intend to use for chat. However, as of now, there is no specific feature to manually select the spell-checking language in the chat interface itself.
For further assistance, consider checking the official Microsoft Teams support resources or community forums for updates on this feature.