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+============
+Localization
+============
+
+.. toctree::
+   :maxdepth: 1
+
+   glossary
+
+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 :doc:`./glossary` for what localization is, and which terms
+we use for what.
+
+Exposing strings
+----------------
+
+Localizers only handle a few file formats in well-known locations in the
+source tree.
+
+The locations are in directories like
+
+    :file:`browser/`\ ``locales/en-US/``\ :file:`subdir/file.ext`
+
+The first thing to note is that only files beneath :file:`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 <https://dxr.mozilla.org/mozilla-central/search?q=path%3Al10n.ini&redirect=true>`_
+in the source code.
+
+An example looks like this
+
+.. code-block:: ini
+
+    [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 :file:`browser/locales/en-US` and
+:file:`browser/branding/official/locales/en-US`, and load more data from
+:file:`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:
+
+.. code-block:: ini
+
+    [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
+builds, :file:`mail/locales/l10n.ini` references
+:file:`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
+------------
+
+This is just a quick overview, please check the
+`XUL Tutorial <https://developer.mozilla.org/docs/Mozilla/Tech/XUL/Tutorial/Localization>`_
+for an in-depth tour.
+
+The following file formats are known to the l10n tool chains:
+
+DTD
+    Used in XUL and XHTML. Also for Android native strings.
+Properties
+    Used from JavaScript and C++. When used from js, also comes with
+    `plural support <https://developer.mozilla.org/docs/Mozilla/Localization/Localization_and_Plurals>`_.
+ini
+    Used by the crashreporter and updater, avoid if possible.
+foo.defines
+    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.
+
+Exceptions
+----------
+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
+:py:mod:`filter.py`, implementing :py:func:`test`, with the following
+signature
+
+.. code-block:: python
+
+    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
+:file:`locales/en-US`. The module is the top-level dir as referenced in
+:file:`l10n.ini`.
+
+For missing strings, the :py:data:`entity` parameter is the key of the string
+in the en-US file.
+
+l10n-merge
+----------
+
+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 :term:`l10n-merge`.
+
+The process is usually triggered via
+
+.. code-block:: bash
+
+    $obj-dir/browser/locales> make merge-de LOCALE_MERGEDIR=$PWD/merge-de
+
+It creates another directory in the object dir, :file:`merge-ab-CD`, in
+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
+in ``en-US``. Thus, for the ``de`` localization of
+:file:`browser/locales/en-US/chrome/browser/browser.dtd`, it checks
+
+1. :file:`$objdir/browser/locales/merge-de/browser/chrome/browser/browser.dtd`
+2. :file:`$(LOCALE_BASEDIR)/de/browser/chrome/browser/browser.dtd`
+3. :file:`browser/locales/en-US/chrome/browser/browser.dtd`
+
+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.
+
+Checks
+------
+
+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.
+
+Android
+^^^^^^^
+
+For Android, we need to localize :file:`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
+:file:`mobile/android/base/locales/en-US/`.
+
+Localizations
+-------------
+
+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/.
+
+You can search inside our localized files on
+`Transvision <https://transvision.mozfr.org/>`_ and
+http://dxr.mozilla.org/l10n-mozilla-aurora/.