
→ Language Preferences → Choose Editing Languages.

You can also reset individual slides to the master style, but this may result in placeholders moving as well, which may be undesirable in some situations.

Although of course in many case this is better done by changing the slide master, a presentation that has had many editors may have lots of 'hard' formatting set which deviates from the underlying master and needs resetting to be consistent. Likewise while you have everything selected you can change other things like fonts, colours etc.
