Fix sphinx-doc#6640: i18n: Failed to override system message translation

Our document describes that users can override system messages via
their own message catalog named `sphinx.mo` under the locale_dirs.
But it has not been used since its beginning of i18n mechanism because
the priority of users' message catalog is lower than system's.

This makes the priority of users' message catalog higher than system's.

CHANGES

@@ -17,6 +17,7 @@ Bugs fixed
 ----------
 
 * #8085: i18n: Add support for having single text domain
+* #6640: i18n: Failed to override system message translation
 * #8093: The highlight warning has wrong location in some builders (LaTeX,
   singlehtml and so on)
 

sphinx/application.py

@@ -293,7 +293,7 @@ def _init_i18n(self) -> None:
                 if catalog.domain == 'sphinx' and catalog.is_outdated():
                     catalog.write_mo(self.config.language)
 
-            locale_dirs = [None, path.join(package_dir, 'locale')] + list(repo.locale_dirs)
+            locale_dirs = [None] + list(repo.locale_dirs) + [path.join(package_dir, 'locale')]
             self.translator, has_translation = locale.init(locale_dirs, self.config.language)
             if has_translation or self.config.language == 'en':
                 # "en" never needs to be translated