Put system locale path after the paths specified by configuration

In Debian, we ship the translation data for Sphinx in the default search
path, /usr/share/locale/. When a .mo file is present there, it takes
priority over the translation files specified by the configuration, so
overriding does not work. This makes test_customize_system_message fail.

sphinx/application.py

@@ -294,8 +294,8 @@ def _init_i18n(self) -> None:
                 if catalog.domain == 'sphinx' and catalog.is_outdated():
                     catalog.write_mo(self.config.language)
 
-            locale_dirs = [None]  # type: List[Optional[str]]
-            locale_dirs += list(repo.locale_dirs)
+            locale_dirs = list(repo.locale_dirs)  # type: List[Optional[str]]
+            locale_dirs += [None]
             locale_dirs += [path.join(package_dir, 'locale')]
 
             self.translator, has_translation = locale.init(locale_dirs, self.config.language)