Localized Builds

Localization repacks

To save on build time, the build system and automation collaborate to allow downloading a packaged en-US Firefox, performing some locale-specific post-processing, and re-packaging a locale-specific Firefox. Such artifacts are termed “single-locale language repacks”. There is another concept of a “multi-locale language build”, which is more like a regular build and less like a re-packaging post-processing step.


These builds rely on make targets that don’t work for artifact builds.

Instructions for single-locale repacks for developers

This assumes that $AB_CD is the locale you want to repack with; you find the available localizations on l10n-central.

  1. You must have a built and packaged object directory, or a pre-built en-US package.

    ./mach build
    ./mach package
  2. Repackage using the locale-specific changes.

    ./mach build installers-$AB_CD

You should find a re-packaged build at OBJDIR/dist/, and a runnable binary in OBJDIR/dist/l10n-stage/. The installers target runs quite a few things for you, including getting the repository for the requested locale from https://hg.mozilla.org/l10n-central/. It will clone them into ~/.mozbuild/l10n-central. If you have an existing repository there, you may want to occasionally update that via hg pull -u. If you prefer to have the l10n repositories at a different location on your disk, you can point to the directory via

ac_add_options --with-l10n-base=/make/this/a/absolute/path

This build also packages a language pack.

Instructions for language packs

Language packs are extensions that contain just the localized resources. Building them doesn’t require an actual build, but they’re only compatible with the mozilla-central source they’re built with.

./mach build langpack-$AB_CD

This target shares much of the logic of the installers-$AB_CD target above, and does the check-out of the localization repository etc. It doesn’t require a package or a build, though. The generated language pack is in OBJDIR/dist/$(MOZ_PKG_PLATFORM)/xpi/.


Despite the platform-dependent location in the build directory, language packs are platform independent, and the content that goes into them needs to be built in a platform-independent way.

Instructions for multi-locale builds

If you want to create a single build with multiple locales, you will do

  1. Create a build and package

    ./mach build
    ./mach package
  2. For each locale you want to include in the build:

    export MOZ_CHROME_MULTILOCALE="de it zh-TW"
       ./mach build chrome-$AB_CD
  3. Create the multilingual package:

    AB_CD=multi ./mach package

General flow of repacks

The general flow of the locale repacks is controlled by $MOZ_BUILD_APP/locales/Makefile.in and toolkit/locales/l10n.mk, plus the packaging build system. The three main entry points above all trigger related build flows:

  1. Get the localization repository, if needed

  2. Run l10n-merge with a prior clobber of the merge dir

  3. Copy l10n files to dist, with minor differences here between l10n-% and chrome-%

  4. Repackage and package

Details on l10n-merge are described in its own section below. The copying of files is mainly controlled by jar.mn, in the few source directories that include localizable files. l10n-% is used for repacks, chrome-% for multi-locale packages. The repackaging is dedicated Python code in toolkit/mozapps/installer/l10n-repack.py, using an existing package. It strips existing chrome l10n resources, and adds localizations and metadata.

Language packs don’t require repackaging. The windows installers are generated by merely packaging an existing repackaged zip into to an installer.

Exposing strings

The localization flow handles a few file formats in well-known locations in the source tree.

Alongside being built by including the directory in $MOZ_BUILD_APP/locales/Makefile.in and respective entries in a jar.mn, we also have configuration files tailored to localization tools and infrastructure. They’re also controlling which files l10n-merge handles, and how.

These configurations are TOML files. They’re part of the bigger localization ecosystem at Mozilla, and the documentation about the file format explains how to set them up, and what the entries mean. In short, you find

    reference = browser/locales/en-US/**
    l10n = {l}browser/**

to add a directory for all localizations. Changes to these files are best submitted for review by :Pike or :flod.

These configuration files are the future, and right now, we still have support for the previous way to configuring l10n, which is described below.

The locations are commonly in directories like


The first thing to note is that only files beneath locales/en-US are exposed to localizers. The second thing to note is that only a few directories are exposed. Which directories are exposed is defined in files called l10n.ini, which are at a few places in the source code.

An example looks like this

depth = ../..

dirs = browser

toolkit = toolkit/locales/l10n.ini

This tells the l10n infrastructure three things:

  • resolve the paths against the directory two levels up

  • include files in browser/locales/en-US and browser/branding/official/locales/en-US

  • load more data from toolkit/locales/l10n.ini

For projects like Thunderbird and SeaMonkey in comm-central, additional data needs to be provided when including an l10n.ini from a different repository:

type = hg
mozilla = mozilla-central
repo = https://hg.mozilla.org/
l10n.ini = toolkit/locales/l10n.ini

This tells the l10n infrastructure where to find the repository, and where inside that repository the l10n.ini file is. This is needed because for local builds, mail/locales/l10n.ini references mozilla/toolkit/locales/l10n.ini, which is where the comm-central build setup expects toolkit to be.

Now that the directories exposed to l10n are known, we can talk about the supported file formats.

File formats

The following file formats are known to the l10n tool chains:


Used in Firefox UI, both declarative and programmatically.


Deprecated. Used in XUL and XHTML.


Used from JavaScript and C++. When used from js, also comes with plural support.


Used by the crashreporter and updater, avoid if possible.


Used during builds, for example to create metadata for language packs or bookmarks.

Adding new formats involves changing various different tools, and is strongly discouraged.


Generally, anything that exists in en-US needs a one-to-one mapping in all localizations. There are a few cases where that’s not wanted, notably around locale configuration and locale-dependent metadata.

For optional strings and files, l10n-merge won’t add en-US content if the localization doesn’t have that content.

For the TOML files, the [[filters]] documentation is a good reference. In short, filters match the localized source code, optionally a key, and an action. An example like

    path = "{l}browser/defines.inc"
    action = "ignore"

indicates that the MOZ_LANGPACK_CONTRIBUTORS in browser/defines.inc is optional.

For the legacy ini configuration files, there’s a Python module filter.py next to the main l10n.ini, implementing test(), with the following signature

def test(mod, path, entity = None):
    if does_not_matter:
        return "ignore"
    if show_but_do_not_merge:
        return "report"
    # default behavior, localizer or build need to do something
    return "error"

For any missing file, this function is called with mod being the module, and path being the relative path inside locales/en-US. The module is the top-level dir as referenced in l10n.ini.

For missing strings, the entity parameter is the key of the string in the en-US file.


The chrome registry in Gecko doesn’t support fallback from a localization to en-US at runtime. Thus, the build needs to ensure that the localization as it’s built into the package has all required strings, and that the strings don’t contain errors. To ensure that, we’re merging the localization and en-US at build time, nick-named l10n-merge.

For Fluent, we’re also removing erroneous messages. For many errors in Fluent, that’s cosmetic, but when a localization has different values or attributes on a message, that’s actually important so that the DOM bindings of Fluent can apply the translation without having to load the en-US source to compare against.

The process can be manually triggered via

$> ./mach build merge-$AB_CD

It creates another directory in the object dir, browser/locales/merge-dir/$AB_CD, in which the sanitized files are stored. The actual repackaging process only looks in the merged directory, so the preparation steps of l10n-merge need to ensure that all files are generated or copied.

l10n-merge modifies a file if it supports the particular file type, and there are missing strings which are not filtered out, or if an existing string shows an error. See the Checks section below for details. If the files are not modified, l10n-merge copies them over to the respective location in the merge dir.


As part of the build and other localization tool chains, we run a variety of source-based checks. Think of them as linters.

The suite of checks is usually determined by file type, i.e., there’s a suite of checks for DTD files and one for properties files, etc.


Now that we talked in-depth about how to expose content to localizers, where are the localizations?

We host a mercurial repository per locale. All of our localizations can be found on https://hg.mozilla.org/l10n-central/.

You can search inside our localized files on Transvision and https://dxr.mozilla.org/l10n-central/source/.