While it is easy to change proofing language for selected text containers, that happens to be a great time waster if you need the language changed across all content in all slides! There are two ways to set the proofing language for your entire presentation — and you can use any of these approaches. Make sure you have the proofing tools installed for all or any of the languages that you need to work within PowerPoint.
Learn how to set proofing language for an entire presentation in PowerPoint 2011 for Mac.

