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.

It's not just Turkey that does the date day/month/year. I believe only the US has the problem of being different.

So the solution is easy: ignore one of the biggest markets in the world and then pretend like the remaining cultures all do things the same.

.NET's support for internationalization extends to things that vary by language and country (they aren't all the same), like names of the days of the week.

i didn't know about the i/I problem (question 3 on their test), that's really neat

Only on Thanksgiving.