The question is, how much effort does it take to make your application pass such tests and how important is it that yours do so?
If you're making an application for filing income taxes in the US, say, I would argue only worrying about usability with ESL speakers and not worrying about how your program would work in Turkey or Japan or whatever.
If you're making an application for filing income taxes in the US, say, I would argue only worrying about usability with ESL speakers and not worrying about how your program would work in Turkey or Japan or whatever.