I use Google Docs in several classes. One problem that comes up a lot is the spell-checker. If my students’ Drive language isn’t English, their English spell-check won’t work. Here’s how to fix it:
With your document open in Google Docs,
Highlight all your text.
Go to the File > Language menu, and
Select English (United States).
Now you should be able to see any incorrectly-spelled English words (and, of course, correct them!)