You can build a multilocale Gaia profile with the following make command:

make profile \
  LOCALE_BASEDIR=locales/ \
  LOCALES_FILE=locales/languages_basecamp.json \
  GAIA_DEFAULT_LOCALE=pt-BR

Use GAIA_PRETRANSLATE=1 to precompile all HTML to include text content in the deafult locale.

The full documentation on building multilocale Gaia and B2G is avaiable on MDN: https://developer.mozilla.org/en-US/docs/Mozilla/Firefox_OS/Building#Building_multilocale

This directory can be used for localization testing. You can put localization files here and Gaia will use them when run in the debug mode.

In order to enable the the debug mode (in a desktop build or in Firefox), run:

$ DEBUG=1 make profile

Files put in this directory should follow the l10n HG repos directory layout.

In the debug mode, Gaia will first try to find the .properties file in

locales/LOCALE/apps/APP/APP.properties

For instance, the localization file for the browser app for French would be located under:

locales/fr/apps/browser/browser.properties

Notice that this way locales/fr can be an HG clone of the French gaia-l10n repository.

If the file does not exist, Gaia will fallback to looking in its regular location, i.e.:

apps/APP/locales/APP.LOCALE.properties

Clone your locale's Gaia l10n repository into this directory. For instace, for French (run from the git clone root):

$ hg clone ssh://hg.mozilla.org/gaia-l10n/fr locales/fr

Create Gaia's profile with:

$ DEBUG=1 make profile

And launch it in a desktop build:

$ b2g-bin -profile profile-debug/

...or in Firefox:

$ firefox -profile profile-debug/

You can now use Gaia and test the localization. When you make changes to the files in the HG clone, just reload the Gaia app to see changes.

Once you're done, commit your changes to HG from the clone you were working in.