Using Office 365 desktop, Windows 10, OneNote.

My primary default language is English. I have installed a Russian language pack, however, when I open a OneNote page with combined English and Russian text, the proofing doesn't work for both languages at the same time.

I have tried Review -> Language -> Set Proofing Language, but can't get OneNote to proof both languages simultaneously. For example:

Привет, как дела? (Hi, how are you?)
Как поживаешь? (how are you - less formal)

Any ideas please?