diff --git a/python/mozbuild/mozpack/packager/l10n.py b/python/mozbuild/mozpack/packager/l10n.py
index c97c6244759..776f689c3d2 100644
--- a/python/mozbuild/mozpack/packager/l10n.py
+++ b/python/mozbuild/mozpack/packager/l10n.py
@@ -19,7 +19,10 @@ from mozpack.packager import (
     SimplePackager,
     SimpleManifestSink,
 )
-from mozpack.files import ManifestFile
+from mozpack.files import (
+    ComposedFinder,
+    ManifestFile,
+)
 from mozpack.copier import (
     FileCopier,
     Jarrer,
@@ -177,9 +180,35 @@ def _repack(app_finder, l10n_finder, copier, formatter, non_chrome=set()):
         copier[path].preload([l.replace(locale, l10n_locale) for l in log])
 
 
-def repack(source, l10n, non_resources=[], non_chrome=set()):
+def repack(source, l10n, extra_l10n={}, non_resources=[], non_chrome=set()):
+    '''
+    Replace localized data from the `source` directory with localized data
+    from `l10n` and `extra_l10n`.
+
+    The `source` argument points to a directory containing a packaged
+    application (in omnijar, jar or flat form).
+    The `l10n` argument points to a directory containing the main localized
+    data (usually in the form of a language pack addon) to use to replace
+    in the packaged application.
+    The `extra_l10n` argument contains a dict associating relative paths in
+    the source to separate directories containing localized data for them.
+    This can be used to point at different language pack addons for different
+    parts of the package application.
+    The `non_resources` argument gives a list of relative paths in the source
+    that should not be added in an omnijar in case the packaged application
+    is in that format.
+    The `non_chrome` argument gives a list of file/directory patterns for
+    localized files that are not listed in a chrome.manifest.
+    '''
     app_finder = UnpackFinder(source)
     l10n_finder = UnpackFinder(l10n)
+    if extra_l10n:
+        finders = {
+            '': l10n_finder,
+        }
+        for base, path in extra_l10n.iteritems():
+            finders[base] = UnpackFinder(path)
+        l10n_finder = ComposedFinder(finders)
     copier = FileCopier()
     if app_finder.kind == 'flat':
         formatter = FlatFormatter(copier)
diff --git a/toolkit/mozapps/installer/l10n-repack.py b/toolkit/mozapps/installer/l10n-repack.py
index bfd88fa492b..783c00b71cb 100644
--- a/toolkit/mozapps/installer/l10n-repack.py
+++ b/toolkit/mozapps/installer/l10n-repack.py
@@ -28,12 +28,23 @@ NON_CHROME = set([
 ])
 
 
+def valid_extra_l10n(arg):
+    if '=' not in arg:
+        raise ValueError('Invalid value')
+    return tuple(arg.split('=', 1))
+
+
 def main():
     parser = ArgumentParser()
     parser.add_argument('build',
                         help='Directory containing the build to repack')
     parser.add_argument('l10n',
                         help='Directory containing the staged langpack')
+    parser.add_argument('extra_l10n', nargs='*', metavar='BASE=PATH',
+                        type=valid_extra_l10n,
+                        help='Extra directories with staged localized files '
+                             'to be considered under the given base in the '
+                             'repacked build')
     parser.add_argument('--non-resource', nargs='+', metavar='PATTERN',
                         default=[],
                         help='Extra files not to be considered as resources')
@@ -41,7 +52,8 @@ def main():
 
     buildconfig.substs['USE_ELF_HACK'] = False
     buildconfig.substs['PKG_SKIP_STRIP'] = True
-    l10n.repack(args.build, args.l10n, args.non_resource, NON_CHROME)
+    l10n.repack(args.build, args.l10n, extra_l10n=dict(args.extra_l10n),
+                non_resources=args.non_resource, non_chrome=NON_CHROME)
 
 
 if __name__ == "__main__":