diff --git a/CHANGES b/CHANGES
index 4c6e4e38d4c..d2e3322c10d 100644
--- a/CHANGES
+++ b/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)
 
diff --git a/sphinx/application.py b/sphinx/application.py
index d84a2c97513..9b8824e69ad 100644
--- a/sphinx/application.py
+++ b/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