If you are just annoyed by the feature, you can disable it in Preferences -> Spelling by unchecking the box for
Check spelling as you type:
You can re-enable it after you're done working with this document.
If you need to keep English spell check, double-check your system language setting in System Preferences -> Language & Text -> Language. I doubt this has changed without you noticing, but there's an off chance PowerPoint may be taking its cue from this setting.
I've always had issues with multiple languages in Microsoft Office, so during installation, I disable all languages except for English. This would be a surefire solution if you don't need the extra languages, but it would require you to uninstall and reinstall Office.
Finally, I would strongly encourage you to look into Keynote ($20 through the App Store) as an alternative to PowerPoint. There would be a bit of a learning curve while you get used to it, but it's quite powerful, the interface is sleek, it runs perfectly, and I find that it's easier to make things "look nice" in Keynote than in PowerPoint. Most U.S. university classrooms will have a video hookup that you can use to run your presentation from a Mac laptop, but if it's not an option for you to run your presentations from your Mac, it can export PowerPoint-compatible files pretty well.
You're not struggling with a Microsoft operating system, so why struggle with Microsoft software?