investigate localized results when locale is not supported

I have added some extra tests to TZP (MOAR to come)

• AFAICT they follow the locale (i.e resolvedOptions)

But not all locales are supported for "everything" - so it falls back to "something", but I cannot determine what that is: so this may be a leak, albeit in edge cases

So, here I am with a Macedonian app language, and Danish navigator.language and Macedonian locale (I used the bug in #41930 (closed) to get in this state), and the xml errors validation messages are returned as English

exhibit-b in the next post