Bug 721482 - Make language codes relocatable

Although all translation files are in the OS X bundle GIMP didn't
recognize other languages than the system's language and English (en_US)
on OS X on other machines. It searched the language code file from the
iso-codes package (iso_639.xml) in the build prefix which is usually not
existing on other machines.
This commit puts that file into the OS X bundle and makes GIMP search
for it there.