Currently, all of the b2g builds on TBPL are using a fixed set of locales which are built into the Gaia repository. These were OK at first, but we're at the point now where we need to be able to test the languages that we'll be initially shipping, as well as provide some way for localizers to test out their work. For these reasons, the following changes will be made to the B2g desktop and device builds:
Unagi, otoro, panda, and the current desktop b2g builds will include 6 locales (instead of the 4 they currently do): Arabic (ar), English (en-US), French (fr), Spanish (es), Brazilian Portuguese (pt-BR), and Mandarin Chinese (zh-TW). These builds are intended for developer consumption and help to test out a wide array of features (rtl languages, languages with long strings, unicode characters, etc).
We will be adding new desktop b2g builds that contain all languages that Gaia is available in (see https://github.com/mozilla-b2g/gaia/blob/master/shared/resources/languages-all.json for the full list). These builds are intended for localizers to see how their translations look and feel.
These changes should be taking place sometime this week. Big thanks to Staś Małolepszy for adding support for this to the Gaia build system.
Note that for now, the Gecko portions (for example, network error pages in the Browser) will not be localized. We will be enabling localization for Gecko as soon as we can, but it's not quite ready yet.