Fix case where language isn't set

sphinx/config.py

@@ -166,7 +166,7 @@ class Config:
 
         # Resolve https://github.com/sphinx-doc/sphinx/issues/10474 where conf.py
         # explicitly sets language to None, by coercing it to English.
-        if namespace["language"] is None:
+        if namespace.get("language", ...) is None:
             namespace["language"] = "en"
 
         return cls(namespace, overrides or {})

tests/test_config.py

@@ -395,3 +395,17 @@ def test_conf_py_language_none(tempdir):
 
     # 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"