Fix #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.

4 files changed, 36 insertions, 3 deletions

diff --git a/CHANGES b/CHANGES
index e30d42e43..481ade0b8 100644
--- a/CHANGES
+++ b/CHANGES
@@ -27,6 +27,7 @@ Bugs fixed
 ----------
 
 * #8085: i18n: Add support for having single text domain
+* #6640: i18n: Failed to override system message translation
 * #8143: autodoc: AttributeError is raised when False value is passed to
   autodoc_default_options
 * #8103: autodoc: functools.cached_property is not considered as a property
diff --git a/sphinx/application.py b/sphinx/application.py
index 385b74d8a..f91027bf7 100644
--- a/sphinx/application.py
+++ b/sphinx/application.py
@@ -18,7 +18,7 @@ import warnings
 from collections import deque
 from io import StringIO
 from os import path
-from typing import Any, Callable, Dict, IO, List, Tuple, Union
+from typing import Any, Callable, Dict, IO, List, Optional, Tuple, Union
 
 from docutils import nodes
 from docutils.nodes import Element, TextElement
@@ -293,7 +293,10 @@ class Sphinx:
                 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]  # type: List[Optional[str]]
+            locale_dirs += list(repo.locale_dirs)
+            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
diff --git a/sphinx/locale/__init__.py b/sphinx/locale/__init__.py
index 385ca3566..5210dc725 100644
--- a/sphinx/locale/__init__.py
+++ b/sphinx/locale/__init__.py
@@ -106,7 +106,7 @@ class _TranslationProxy(UserString):
 translators = defaultdict(NullTranslations)  # type: Dict[Tuple[str, str], NullTranslations]
 
 
-def init(locale_dirs: List[str], language: str,
+def init(locale_dirs: List[Optional[str]], language: str,
          catalog: str = 'sphinx', namespace: str = 'general') -> Tuple[NullTranslations, bool]:
     """Look for message catalogs in `locale_dirs` and *ensure* that there is at
     least a NullTranslations catalog set in `translators`. If called multiple
diff --git a/tests/test_intl.py b/tests/test_intl.py
index 1d1282baa..c0b87d5ce 100644
--- a/tests/test_intl.py
+++ b/tests/test_intl.py
@@ -14,8 +14,10 @@ import re
 
 import pytest
 from babel.messages import pofile, mofile
+from babel.messages.catalog import Catalog
 from docutils import nodes
 
+from sphinx import locale
 from sphinx.testing.util import (
     path, etree_parse, strip_escseq,
     assert_re_search, assert_not_re_search, assert_startswith, assert_node
@@ -1289,3 +1291,30 @@ def test_image_glob_intl_using_figure_language_filename(app):
 
 def getwarning(warnings):
     return strip_escseq(warnings.getvalue().replace(os.sep, '/'))
+
+
+@pytest.mark.sphinx('html', testroot='basic', confoverrides={'language': 'de'})
+def test_customize_system_message(make_app, app_params, sphinx_test_tempdir):
+    try:
+        # clear translators cache
+        locale.translators.clear()
+
+        # prepare message catalog (.po)
+        locale_dir = sphinx_test_tempdir / 'basic' / 'locales' / 'de' / 'LC_MESSAGES'
+        locale_dir.makedirs()
+        with (locale_dir / 'sphinx.po').open('wb') as f:
+            catalog = Catalog()
+            catalog.add('Quick search', 'QUICK SEARCH')
+            pofile.write_po(f, catalog)
+
+        # construct application and convert po file to .mo
+        args, kwargs = app_params
+        app = make_app(*args, **kwargs)
+        assert (locale_dir / 'sphinx.mo').exists()
+        assert app.translator.gettext('Quick search') == 'QUICK SEARCH'
+
+        app.build()
+        content = (app.outdir / 'index.html').read_text()
+        assert 'QUICK SEARCH' in content
+    finally:
+        locale.translators.clear()