How do you test your app for Iñtërnâtiônàlizætiøn? (Internationalization?)

Pick a culture where the text reads from right to left and set your system up for that - make sure that it reads properly (easier said than done...).

Use one of the three "pseudo-locales" available since Windows Vista:

The three different pseudo-locale are for testing 3 kinds of locales:

Base The qps-ploc locale is used for English-like pseudo localizations. Its strings are longer versions of English strings, using non-Latin and accented characters instead of the normal script. Additionally simple Latin strings should sort in reverse order with this locale.

Mirrored qpa-mirr is used for right-to-left pseudo data, which is another area of interest for testing.

East Asian qps-asia is intended to utilize the large CJK character repertoire, which is also useful for testing.

Windows will start formatting dates, times, numbers, currencies in a made-up psuedo-locale that looks enough like english that you can work with it, but obvious enough when you're not respecting the locale:

[Шěđлеśđαỳ !!!], 8 ōf [Μäŕςћ !!] ōf 2006

There is more to internationalization than unicode handling. You also need to make sure that dates show up localized to the user's timezone, if you know it (and make sure there's a way for people to tell you what their time zone is).

One handy fact for testing timezone handling is that there are two timezones (Pacific/Tongatapu and Pacific/Midway) that are actually 24 hours apart. So if timezones are being handled properly, the dates should never be the same for users in those two timezones for any timestamp. If you use any other timezones in your tests, results may vary depending on the time of day you run your test suite.

You also need to make sure dates and times are formatted in a way that makes sense for the user's locale, or failing that, that any potential ambiguity in the rendering of dates is explained (e.g. "05/11/2009 (dd/mm/yyyy)").