I often need to write emails in French that contain English technical words and it seems that the spell check in OSX can't understand that a document can contain both languages.
The result is either French or English words are red-underlined and this isn't pretty.
I've tried the trick on How to make Firefox to spellcheck in multiple languages simultaneously? that consists in merging several .dic and .aff files downloaded from titoBouzout/Dictionaries but the grammar check is less precise than the default OSX one.