Coerce language = None in conf.py to Engligh

sphinx/config.py

@@ -163,6 +163,12 @@ def read(cls, confdir: str, overrides: Dict = None, tags: Tags = None) -> "Confi
             raise ConfigError(__("config directory doesn't contain a conf.py file (%s)") %
                               confdir)
         namespace = eval_config_file(filename, tags)
+
+        # Resolve https://github.com/sphinx-doc/sphinx/issues/10474 where conf.py
+        # explicitly sets language to None, by coercing it to English.
+        if namespace.get("language", ...) is None:
+            namespace["language"] = "en"
+
         return cls(namespace, overrides or {})
 
     def convert_overrides(self, name: str, value: Any) -> Any:

tests/test_config.py

@@ -381,3 +381,31 @@ def test_nitpick_ignore_regex_fullmatch(app, status, warning):
     assert len(warning) == len(nitpick_warnings)
     for actual, expected in zip(warning, nitpick_warnings):
         assert expected in actual
+
+
+def test_conf_py_language_none(tempdir):
+    """Regression test for #10474."""
+
+    # Given a conf.py file with language = None
+    (tempdir / 'conf.py').write_text("language = None", encoding='utf-8')
+
+    # When we load conf.py into a Config object
+    cfg = Config.read(tempdir, {}, None)
+    cfg.init_values()
+
+    # Then the language is coerced to English
+    assert cfg.language == "en"
+
+
+def test_conf_py_no_language(tempdir):
+    """Regression test for #10474."""
+
+    # Given a conf.py file with no language attribute
+    (tempdir / 'conf.py').write_text("", encoding='utf-8')
+
+    # When we load conf.py into a Config object
+    cfg = Config.read(tempdir, {}, None)
+    cfg.init_values()
+
+    # Then the language is coerced to English
+    assert cfg.language == "en"