Plus, it's got a toolchain. GNU gettext msggen and friends work with it, so you can extract (and update) localizable strings, and have your translators use whatever tools they alrady like for dealing with PO files.

(One note: if you define a gettext()/_()-like function for use in code, don't name it _ because there's a funky problem with the meaning of it. Use __ (double underscore); Autrijus' version of gettext is patched to handle it.)