1 files changed, 100 insertions, 0 deletions
diff --git a/build/docs/locales.rst b/build/docs/locales.rst
new file mode 100644
index 000000000..4705ed963
--- /dev/null
+++ b/build/docs/locales.rst
@@ -0,0 +1,100 @@
+.. _localization:
+
+===================
+Localization (l10n)
+===================
+
+Single-locale language 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.
+
+There are scripts in-tree in mozharness to orchestrate these re-packaging
+steps for `Desktop
+<https://dxr.mozilla.org/mozilla-central/source/testing/mozharness/scripts/desktop_l10n.py>`_
+and `Android
+<https://dxr.mozilla.org/mozilla-central/source/testing/mozharness/scripts/mobile_l10n.py>`_
+but they rely heavily on buildbot information so they are almost impossible to
+run locally.
+
+The following instructions are extracted from the `Android script with hg hash
+494289c7
+<https://dxr.mozilla.org/mozilla-central/rev/494289c72ba3997183e7b5beaca3e0447ecaf96d/testing/mozharness/scripts/mobile_l10n.py>`_,
+and may need to be updated and slightly modified for Desktop.
+
+Step by step instructions for Android
+-------------------------------------
+
+This assumes that ``$AB_CD`` is the locale you want to repack with; I tested
+with "ar" and "en-GB".
+
+.. warning:: l10n repacks do not work with artifact builds.  Repackaging
+   compiles no code so supporting ``--disable-compile-environment`` would not
+   save much, if any, time.
+
+#. You must have a built and packaged object directory, or a pre-built
+   ``en-US`` package.
+
+   .. code-block:: shell
+
+      ./mach build
+      ./mach package
+
+#. Clone ``l10n-central/$AB_CD`` so that it is a sibling to your
+   ``mozilla-central`` directory.
+
+   .. code-block:: shell
+
+      $ ls -al
+      mozilla-central
+      ...
+      $ mkdir -p l10n-central
+      $ hg clone https://hg.mozilla.org/l10n-central/$AB_CD l10n-central/$AB_CD
+      $ ls -al
+      mozilla-central
+      l10n-central/$AB_CD
+      ...
+
+#. Copy your ``mozconfig`` to ``mozconfig.l10n`` and add the following.
+
+   ::
+
+      ac_add_options --with-l10n-base=../../l10n-central
+      ac_add_options --disable-tests
+      mk_add_options MOZ_OBJDIR=./objdir-l10n
+
+#. Configure and prepare the l10n object directory.
+
+   .. code-block:: shell
+
+      MOZCONFIG=mozconfig.l10n ./mach configure
+      MOZCONFIG=mozconfig.l10n ./mach build -C config export
+      MOZCONFIG=mozconfig.l10n ./mach build buildid.h
+
+#. Copy your built package and unpack it into the l10n object directory.
+
+   .. code-block:: shell
+
+      cp $OBJDIR/dist/fennec-*en-US*.apk ./objdir-l10n/dist
+      MOZCONFIG=mozconfig.l10n ./mach build -C mobile/android/locales unpack
+
+#. Run the ``compare-locales`` script to write locale-specific changes into
+   ``objdir-l10n/merged``.
+
+   .. code-block:: shell
+
+      MOZCONFIG=mozconfig.l10n ./mach compare-locales --merge-dir objdir-l10n/merged $AB_CD
+
+#. Finally, repackage using the locale-specific changes.
+
+   .. code-block:: shell
+
+      MOZCONFIG=mozconfig.l10n LOCALE_MERGEDIR=`realpath objdir-l10n/merged` ./mach build -C mobile/android/locales installers-$AB_CD
+
+   (Note the absolute path for ``LOCALE_MERGEDIR``.)  You should find a
+   re-packaged build at ``objdir-l10n/dist/fennec-*$AB_CD*.apk``.