Most scripts will mostly work if you support localization, but have only tested English. You'll most likely find issues with scripts needing compulsory ligatures (Devanagari) and CJK; so test them to make sure that someone trying to hack in an alternate localization doesn't run into a missing font, bad line breaking, unreadable text size, etc.

[Edit: "first" meaning "before releasing", not meaning "before other scripts."]

It'd be good to test with Right to Left languages as well. That's a big one that can mess things up in unexpected ways.