diff --git a/CHANGES b/CHANGES
index 36177d28ba..87b35403da 100644
--- a/CHANGES
+++ b/CHANGES
@@ -7,6 +7,9 @@ Dependencies
 Incompatible changes
 --------------------
 
+* #10474: :confval:`language` does not accept ``None`` as it value.  The default
+  value of ``language`` becomes to ``'en'`` now.
+
 Deprecated
 ----------
 
diff --git a/doc/usage/configuration.rst b/doc/usage/configuration.rst
index 677eb39271..f297e5c5cb 100644
--- a/doc/usage/configuration.rst
+++ b/doc/usage/configuration.rst
@@ -725,7 +725,7 @@ documentation on :ref:`intl` for details.
    (e.g. the German version of ``myfigure.png`` will be ``myfigure.de.png``
    by default setting) and substitute them for original figures.  In the LaTeX
    builder, a suitable language will be selected as an option for the *Babel*
-   package.  Default is ``None``, which means that no translation will be done.
+   package.  Default is ``'en'``.
 
    .. versionadded:: 0.5
 
@@ -733,6 +733,8 @@ documentation on :ref:`intl` for details.
 
       Support figure substitution
 
+   .. versionchanged:: 5.0
+
    Currently supported languages by Sphinx are:
 
    * ``ar`` -- Arabic
@@ -745,7 +747,7 @@ documentation on :ref:`intl` for details.
    * ``da`` -- Danish
    * ``de`` -- German
    * ``el`` -- Greek
-   * ``en`` -- English
+   * ``en`` -- English (default)
    * ``eo`` -- Esperanto
    * ``es`` -- Spanish
    * ``et`` -- Estonian
diff --git a/sphinx/config.py b/sphinx/config.py
index 5f92479d38..15337e924d 100644
--- a/sphinx/config.py
+++ b/sphinx/config.py
@@ -163,6 +163,17 @@ 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)
+
+        # Note: Old sphinx projects have been configured as "langugae = None" because
+        #       sphinx-quickstart previously generated this by default.
+        #       To keep compatibility, they should be fallback to 'en' for a while
+        #       (This conversion should not be removed before 2025-01-01).
+        if namespace.get("language", ...) is None:
+            logger.warning(__("Invalid configuration value found: 'language = None'. "
+                              "Update your configuration to a valid langauge code. "
+                              "Falling back to 'en' (English)."))
+            namespace["language"] = "en"
+
         return cls(namespace, overrides or {})
 
     def convert_overrides(self, name: str, value: Any) -> Any:
diff --git a/tests/test_config.py b/tests/test_config.py
index 3d72a6b0f0..c0b8864e00 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -381,3 +381,49 @@ 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"
+
+
+@mock.patch("sphinx.config.logger")
+def test_conf_py_language_none_warning(logger, 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
+    Config.read(tempdir, {}, None)
+
+    # Then a warning is raised
+    assert logger.warning.called
+    assert logger.warning.call_args[0][0] == (
+        "Invalid configuration value found: 'language = None'. "
+        "Update your configuration to a valid langauge code. "
+        "Falling back to 'en' (English).")
+
+
+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"