On Interpr.it, developers upload their extensions, translators localize them into their native language, and then developers can download the new locale files to use in their next extension update.
Interpr.it for Developers
Developer interaction is limited to the upload and download of locale files. In fact, developers can interact with Interpr.it completely from the command line via the /api/upload and /api/download API methods. (There are also API methods for translating messages and retrieving the translation history of a message.)
Interpr.it for Translators
Once an extension has been uploaded to Interpr.it, a status page is generated that shows the progress made on all of the locales.
Each locale code links to a translation page, which is a listing of all of the messages (or "phrases" or "strings") used in the extension.
Sign in/out is tied to the Mozilla Persona login system
The Interpr.it site itself is localized using itself. (Interpr.it obviously isn’t a browser extension, but it does use Google Chrome-style JSON locale files, so it’s compatible with Interpr.it’s translation system.) To access a localized version of Interpr.it, select a locale code from the menu in top-right corner of the website, or manually type a URL like es.interpr.it.