How do I use spell check in OSX with several languages?
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.