Add test

tests/test_config.py

@@ -381,3 +381,17 @@ 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"