With Power-user, you can very easily change the autocorrect language for the entire presentation.

The issue with the PowerPoint language autocorrect

PowerPoint lets you change the autocorrect language from the status bar at the bottom of the PowerPoint window. But unfortunately, this will only change the language for the currently selected shape, nothing more. 

Imagine if you are translating a presentation from English to French. All the text in all the shapes will be underlined in red as if it all contained grammar or spelling errors, just because all these shapes still have English as autocorrect language, even though everything is in French as you progress through the translation. And of course you don't want to manually change the language for each and every shape in the presentation. 

So this behavior makes the whole feature useless and your slides harder to read because of this red underline everywhere.

Changing the autocorrect language in the entire PowerPoint presentation

To change the language in an entire PowerPoint presentation:

• From the "Text" group, at the right of the flag icon click the arrow to expend the menu of languages
• In this menu, click the language to apply to the entire presentation.

This will convert the entire presentation to the chosen autocorrect language, applying it on every object on all slides.

This feature can be customized so that you always keep your favorite languages just 1 click away. 

• Click the Language drop-down arrow.
• The below window will open:

• On the left you can see the list of languages that will appear in your Language menu. The one on the very top can also be clicked directly from the ribbon.
• Select a language from the left and click Up or Down to move it in the list. Click Remove to remove it from the list of your favorite languages. 
• Click another language from the right part of the window to add it to the list.