The documentation here is targeted at developers, writing localizable code for Firefox and Firefox for Android, as well as Thunderbird and SeaMonkey.
If you haven’t dealt with localization in gecko code before, it’s a good idea to check the Glossary for what localization is, and which terms we use for what.
Localizers only handle a few file formats in well-known locations in the source tree.
The locations are in directories like
The first thing to note is that only files beneath
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
in the source code.
An example looks like this
[general] depth = ../.. [compare] dirs = browser browser/branding/official [includes] 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/branding/official/locales/en-US, and load more data from
For projects like Thunderbird and SeaMonkey in
data needs to be provided when including an
l10n.ini from a different
[include_toolkit] type = hg mozilla = mozilla-central repo = http://hg.mozilla.org/ l10n.ini = toolkit/locales/l10n.ini
This tells the l10n pieces where to find the repository, and where inside
that repository the
l10n.ini file is. This is needed because for local
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.
This is just a quick overview, please check the XUL Tutorial for an in-depth tour.
The following file formats are known to the l10n tool chains:
- Used in XUL and XHTML. Also for Android native strings.
- Used by the crashreporter and updater, avoid if possible.
- Used during builds, for example to create file:install.rdf for language packs.
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 search settings and spell-checking dictionaries.
To enable tools to adjust to those exceptions, there’s a python-coded
test(), with the following
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
the module, and
path being the relative path inside
locales/en-US. The module is the top-level dir as referenced in
For missing strings, the
entity parameter is the key of the string
in the en-US file.
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
at build time, nick-named l10n-merge.
The process is usually triggered via
$obj-dir/browser/locales> make merge-de LOCALE_MERGEDIR=$PWD/merge-de
It creates another directory in the object dir,
which the modified files are stored. The actual repackaging process looks for
the localized files in the merge dir first, then the localized file, and then
en-US. Thus, for the
de localization of
browser/locales/en-US/chrome/browser/browser.dtd, it checks
and will include the first of those files it finds.
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.
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. An exception are Android-specific checks.
For Android, we need to localize
strings.xml. We’re doing so via DTD
files, which is mostly OK. But the strings inside the XML file have to
satisfy additional constraints about quotes etc, that are not part of XML.
There’s probably some historic background on why things are the way they are.
The Android-specific checks are enabled for DTD files that are in
Now that we talked in-depth about how to expose content to localizers, where are the localizations?
We host a mercurial repository per locale and per branch. Most of our localizations only work starting with aurora, so the bulk of the localizations is found on https://hg.mozilla.org/releases/l10n/mozilla-aurora/. We have several localizations continuously working with mozilla-central, those repositories are on https://hg.mozilla.org/l10n-central/.