Additional settings-related features and issues within Microsoft Teams for business
Hello @Eamon Nerbonne,
In addition to the suggestions provided by Oliver Bennett, please try updating the preferred languages associated with your work account.
First, sign out of Teams, then open My Account – Settings & Privacy. Under Preferred languages, select "Add a language", choose the language you want to use, and save the changes.
Afterward, sign back in to Teams, switch to the secondary tenant, and check whether the spellcheck language can now be added and retained.
If you cannot add a language from the My Account page due to insufficient permissions, please contact the IT administrator of the secondary tenant and ask whether they can help update or enable the language preference for your account.
Please let me know whether this changes the behavior.