Hi Giuseppe,
Thank you very much for the question!
On Windows, Spell Checker uses Microsoft Word’s Spelling Check, which means that you must have Microsoft Word installed on your computer together with the dictionary of the language you are using. On macOS, Spell Checker uses macOS system preferences.
You can find more information about the Spell Checker
here.
As Laszlo mentioned, the Spell Checker functionality does not depend on whether the project is a solo or a teamwork project. But let me know if the article above helps to solve your issue!
Best regards,
Minh
Minh Nguyen
Technical Support Engineer
GRAPHISOFT