Are you sure it is the Zimbra spell checker and not the browser spell checker? I've had users who have their browser setting to one language and their Zimbra setting to another.
If it is a red underline under what it thinks are misspelled words, it sounds like the browser is set to the wrong language for spell checking, not the Zimbra client. The Zimbra client typically displays misspelled words as highlighted in yellow.
Note: I'm still on Zimbra 7.x so I don't know if 8.x changed how it does spell check.
State University of New York at New Paltz