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.
Best Answer
Go to system preferences/keyboard/text/spelling and click on the window and go to the Setup item. Then check the boxes for only the two languages you want and set it to Automatic. Also set the dictionary to Automatic in Edit > Spelling and Grammar > Show Spelling and Grammar.
I suspect this works better for separate sentences or paragraphs in different languages than for different languages in one sentence.