How do I change the language of all Powerpoint slides at once?

To change the language of the entire PowerPoint easily, open the View tab and select the Outline view.

Now press

• Ctrl+A to select all.
• Tools → Language → Choose your language to set.

Likewise while you have everything selected you can change other things like fonts, colours etc. 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. 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.

PowerPoint 2013

• View → Outline → select all slides (in a left menu) via Ctrl+A.
• Review → Language → Set Proofing Language... → Choose your language to set.

As for me - PowerPoint restart was needed. Probably because I also did changed Editing Language:

• Review → Language → Set Proofing Language... → Language Preferences → Choose Editing Languages.

Using Powerpoint 2010 I opened the Outline menu -

Selected all text (Ctrl+A), opened the language menu and set my proofing language

And it worked!

The language menu is located on the Review ribbon tab (after the Slide Show tab and not visible on the screenshot).