What should I set this to?

You must set it to match the language setting of Apple Notes. This means setting to the same language setting as your OS, unless you configured an app-specific language for Notes.

Open Language & Region in Settings.

You should set ProNotes’ Language Setting option to match the first language under Preferred Languages.

If you have an app-specific language for Notes, disregard what was previously mentioned and set the Language Setting to match Notes’.

I don’t see my language under ProNote’s Language Settings

If you do not see your language in the ProNotes’ Language Settings dropdown, such as in the case of English (New Zealand), that means that Apple Notes doesn’t support that language and is falling back to some other language.

You can find the real language Apple Notes is using by adding Notes under Language & Region > Applications:

We can see that Apple Notes falls back to English (Australia) when the user’s language is English (New Zealand). Click on Cancel.

Set ProNotes’ Language Setting to match Notes’.