I used character styles that set the proofing language with hot keys assigned, so shift-alt-1 sets to English, shift-alt-2 to German, etc. As character styles they apply both to the current insertion point when typing or any selected range (e.g. when I forgot to set it proactively and now have a line spattered with wiggles)

Or just set the proofing language for the entire text to None to banish all spelling and grammar diagnostics.