If you haven't yet upgraded to version 1.5, please see this section for translation on earlier releases.
There's a built-in Locale Editor in the Filetransfer appliance. It's available in Admin → Locale Editor. From there, you can add, edit and upload new language files. Most of the times, that's all you need to do.
The only caveat is if you want to override any of the built-in locales.
The translation system tries to be as smart as possible. It will fall back in multiple steps to base English. As an example, lets say that a Swiss French localization is added. It will have the base name of fr-CH. The translation system will search, in order:
The upshot of this is that it's safe, the system will always find a string to use, and that it's easy to just override the strings that are needed.
If you want to override the english translation, please create a new language with key: en-XX and name: English (override). Anything you add in this file will override the English translation, and anything missing will fall back to English.
Say that you want to use Email as the Login label on the home page, you can search for “Login Label” and add “Email” to that field and save. Same for any other string that you want to override.
The same applies for other languages, say French. You can create a fr-XX language that overrides the French default translation. The only thing that wouldn't work is if you wanted to override a subtranslation like fr-CH. That can't be furhter overwridden.