I write mails in two languages. When I write in my native language, and want to use the spelling checker, I have to go through the following sequence of steps:
If you change the language selection, it should immediately rerun the spelling checker. Turning it off and back on again may be a time-tested IT meme, but it's not appropriate as a necessary step in a user interface.
If I may suggest a radical option, why not remove the "check spelling" menu altogether? Just have the language menu. If no languages are selected, no spelling is checked. If you select one, checking is turned on.