Localizing Firefox is as simple as installing a xpi for that language and setting it as your default locale. The difficulty is in localizing the Windows and Unix installers, legacy applications. Each localized build not only has the separate locale xpi, it sets that locale to be that build's default, and configures the installer with a complete set of strings for that locale that are inserted at compile-time.
If you want to help update our installer software, contact me at cmp at m.o (I'm the tech lead for Mozilla releases). I can point you in the right direction to get started. Being proficient in C, Unix and Windows programming would make the task easier.