--- a/python/mozbuild/mozpack/packager/l10n.py
+++ b/python/mozbuild/mozpack/packager/l10n.py
@@ -211,16 +211,22 @@ def _repack(app_finder, l10n_finder, cop
 
     # Add any remaining non chrome files.
     for pattern in non_chrome:
         for base in bases:
             for p, f in l10n_finder.find(mozpath.join(base, pattern)):
                 if not formatter.contains(p):
                     formatter.add(p, f)
 
+    # Resources in `localization` directories are packaged from the source and then
+    # if localized versions are present in the l10n dir, we package them as well
+    # keeping the source dir resources as a runtime fallback.
+    for p, f in l10n_finder.find('**/localization'):
+        formatter.add(p, f)
+
     # Transplant jar preloading information.
     for path, log in app_finder.jarlogs.iteritems():
         assert isinstance(copier[path], Jarrer)
         copier[path].preload([l.replace(locale, l10n_locale) for l in log])
 
 
 def repack(source, l10n, extra_l10n={}, non_resources=[], non_chrome=set()):
     '''