Hello Chris, I'll explain all procedure step by step.
I've got two language keyboard (English + Russian), I don't like some symbols mapping on Mac keyboard, so what I'm doing:
1) Download "Ukelele_2.2.8.dmg"
- Choose Russian keyboard as default input source in OS X
- In Ukelele 'File' -> 'New from current input source'
- Remap any key (for example remap '=' with '~' or any key)
- In Ukelele 'File' -> 'Save' (you will get 'FileName.keylayout' file)
- Move this file to '/Library/Keyboard Layouts'
- Open OS X 'System Preferences' -> 'Keyboard' -> 'Input Sources'
- Press '+' at the bottom of the window to add new input source
- Choose group 'Others' and pick our 'FileName' (custom layout we've just created)
- Choose our custom input source (Cmd + Space or mouse clicking on top right corner of yor mac screen with flag)
- Launch 'OneNote' and start typing
Result:
All words you're typing with custom input source will be underlined with spellchecker, as OneNote can't recognise language from custom input source it will choose system default input source (which for me is English), but because I'm typing with Russian
input source all word got underlined.
This custom input source works fine with all Microsoft Office 2011 applications, and all other applications in OS X, but OneNote doesn't work fine. So please contact 'Office 2011 for Mac' developers, they will explain to OneNote developers how to do spellchecking
correct way!
P.S. For testing you can replace Russian with any language you want, but not English (while English will remain your default input source in OS X Settings)
P.P.S. As Microsoft doesn't fix this bug for 6 months I recommend everyone switch to any better solution with correct spellcheck. As for myself I got that Microsoft can't write soft for Mac at all.