I love OneNote for many reasons, and spend a lot of time in it every day for my work.
Changing the editing language ("set proofing language") is a big hassle. It would be MUCH easier if it were like Word, with the commonly used languages at the top of the window so you don't have to scroll down the huge list (or pinned favorite languages, to make it even better).
I have tried some typical workarounds but they don't work consistently...
up/down arrow keys don't move from where you see, but actually starts at the previous point where you were, far away in the list
typing the name of the language, same thing, often starts at another place in the list and you have to mouse over and click on it to activate
I work in 2 languages, so some pages are in English, others are in Spanish. It constantly defaults back to one instead of making it easier for each page to have its own language in a more intuitive way.
And what about the option to record a Macro? That would help for this and other repetitive tasks in OneNote.