Turning on Multilingual Spellcheck
Click on the Review tab to pull up the Review ribbon
Click on the Language button
Choose "Set Proofing Language"
Scroll down in the new window that appears and choose the language you'd like to proof in then click OK
You may have to scroll further down than you think to get to the languages with proofing options
Make sure you choose a language with the ABC checkmark next to it.
You may need to select all the text in your document and repeat this process to apply the language selection to existing text.