How to stop PowerPoint from being "smart" and constantly changing proofing language?

Solution 1:

I managed to solve this! It is not a PP problem, it depends on the input language keyboard. You are using the English keyboard in Windows 10 and that is why it keeps "smart" changing it to English. But you can change the keyboard of your preferred language.

1. Click on keyboard language on the bottom right part of the screen (ENG)
2. Language preferences
3. Under Preferred languages click Russian and then options
4. Add keyboard -> English (I guess your physical keyboard is English)
5. Delete the Russian keyboard from the Russian language
6. Now go back and move Russian as TOP preferred language

Now you are writing in Russian but the keyboard is English! So PowerPoint won't change it when you set it to be default Russian in PowerPoint!