diff --git a/news/default-language-controlpanel-sync.bugfix b/news/default-language-controlpanel-sync.bugfix
new file mode 100644
index 000000000..c0e3255a9
--- /dev/null
+++ b/news/default-language-controlpanel-sync.bugfix
@@ -0,0 +1 @@
+Patching `@controlpanels/language` now syncs the site language on non-multilingual sites when `default_language` changes, while multilingual sites keep their existing language state. @sneridagh
diff --git a/src/plone/restapi/deserializer/controlpanels/configure.zcml b/src/plone/restapi/deserializer/controlpanels/configure.zcml
index e6202ceb3..8fb2b3fe4 100644
--- a/src/plone/restapi/deserializer/controlpanels/configure.zcml
+++ b/src/plone/restapi/deserializer/controlpanels/configure.zcml
@@ -5,5 +5,6 @@
 
   <adapter factory=".ControlpanelDeserializeFromJson" />
   <adapter factory=".types.DexterityTypesControlpanelDeserializeFromJson" />
+  <adapter factory=".deserializers.ControlpanelLanguageFieldDeserializer" />
 
 </configure>
diff --git a/src/plone/restapi/deserializer/controlpanels/deserializers.py b/src/plone/restapi/deserializer/controlpanels/deserializers.py
new file mode 100644
index 000000000..79d5da746
--- /dev/null
+++ b/src/plone/restapi/deserializer/controlpanels/deserializers.py
@@ -0,0 +1,45 @@
+from plone.dexterity.interfaces import IDexterityContent
+from plone.i18n.interfaces import ILanguageSchema
+from plone.restapi import HAS_MULTILINGUAL
+from plone.restapi.deserializer.dxfields import ChoiceFieldDeserializer
+from plone.restapi.interfaces import IControlpanelLayer
+from plone.restapi.interfaces import IFieldDeserializer
+from zope.component import adapter
+from zope.component.hooks import getSite
+from zope.interface import implementer
+from zope.schema.interfaces import IChoice
+
+if HAS_MULTILINGUAL:
+    from plone.app.multilingual.interfaces import IPloneAppMultilingualInstalled
+
+
+@implementer(IFieldDeserializer)
+@adapter(IChoice, IDexterityContent, IControlpanelLayer)
+class ControlpanelLanguageFieldDeserializer(ChoiceFieldDeserializer):
+    def __call__(self, value):
+        value = super().__call__(value)
+
+        if (
+            self.field.interface is ILanguageSchema
+            and self.field.getName() == "default_language"
+        ):
+            self._sync_site_language(value)
+
+        return value
+
+    def _sync_site_language(self, language):
+        if not IControlpanelLayer.providedBy(self.request):
+            return
+
+        if HAS_MULTILINGUAL and IPloneAppMultilingualInstalled.providedBy(self.request):
+            return
+
+        portal = getSite()
+        if portal is None:
+            return
+
+        if portal.Language() == language:
+            return
+
+        portal.setLanguage(language)
+        self.request["HTTP_ACCEPT_LANGUAGE"] = language
diff --git a/src/plone/restapi/tests/test_services_controlpanel_language.py b/src/plone/restapi/tests/test_services_controlpanel_language.py
new file mode 100644
index 000000000..7b49a29f1
--- /dev/null
+++ b/src/plone/restapi/tests/test_services_controlpanel_language.py
@@ -0,0 +1,151 @@
+from plone.app.testing import setRoles
+from plone.app.testing import SITE_OWNER_NAME
+from plone.app.testing import SITE_OWNER_PASSWORD
+from plone.app.testing import TEST_USER_ID
+from plone.i18n.interfaces import ILanguageSchema
+from plone.registry.interfaces import IRegistry
+from plone.restapi import HAS_MULTILINGUAL
+from plone.restapi.testing import PLONE_RESTAPI_DX_FUNCTIONAL_TESTING
+from plone.restapi.testing import PLONE_RESTAPI_DX_PAM_FUNCTIONAL_TESTING
+from plone.restapi.testing import RelativeSession
+from zope.component import getUtility
+
+import transaction
+import unittest
+
+if HAS_MULTILINGUAL:
+    from plone.app.multilingual.interfaces import IPloneAppMultilingualInstalled
+
+
+class TestLanguageControlpanel(unittest.TestCase):
+
+    layer = PLONE_RESTAPI_DX_FUNCTIONAL_TESTING
+
+    def setUp(self):
+        self.app = self.layer["app"]
+        self.portal = self.layer["portal"]
+        self.request = self.layer["request"]
+        self.portal_url = self.portal.absolute_url()
+        setRoles(self.portal, TEST_USER_ID, ["Manager"])
+
+        self.api_session = RelativeSession(self.portal_url, test=self)
+        self.api_session.headers.update({"Accept": "application/json"})
+        self.api_session.auth = (SITE_OWNER_NAME, SITE_OWNER_PASSWORD)
+
+    def tearDown(self):
+        self.api_session.close()
+
+    def test_update_language_syncs_site_language(self):
+        registry = getUtility(IRegistry)
+        language_settings = registry.forInterface(
+            ILanguageSchema, prefix="plone", check=False
+        )
+        old_default_language = language_settings.default_language
+        old_site_language = self.portal.Language()
+        new_language = "de"
+
+        try:
+            self.portal.setLanguage("en")
+            language_settings.default_language = "en"
+            transaction.commit()
+
+            response = self.api_session.patch(
+                "/@controlpanels/language",
+                json={"default_language": new_language},
+            )
+            transaction.begin()
+
+            self.assertEqual(204, response.status_code)
+            self.assertEqual(new_language, language_settings.default_language)
+            self.assertEqual(new_language, self.portal.Language())
+        finally:
+            language_settings.default_language = old_default_language
+            self.portal.setLanguage(old_site_language)
+            transaction.commit()
+
+    def test_update_non_language_choice_field_does_not_sync_site_language(self):
+        old_site_language = self.portal.Language()
+
+        try:
+            self.portal.setLanguage("en")
+            transaction.commit()
+            response = self.api_session.patch(
+                "/@controlpanels/editing",
+                json={"default_editor": "TinyMCE"},
+            )
+            transaction.begin()
+
+            self.assertEqual(204, response.status_code)
+            self.assertEqual("en", self.portal.Language())
+        finally:
+            self.portal.setLanguage(old_site_language)
+            transaction.commit()
+
+    def test_update_invalid_language_does_not_sync_site_language(self):
+        old_site_language = self.portal.Language()
+
+        try:
+            self.portal.setLanguage("en")
+            transaction.commit()
+            response = self.api_session.patch(
+                "/@controlpanels/language",
+                json={"default_language": "not-a-language"},
+            )
+            transaction.begin()
+
+            self.assertEqual(400, response.status_code)
+            self.assertEqual("en", self.portal.Language())
+        finally:
+            self.portal.setLanguage(old_site_language)
+            transaction.commit()
+
+
+@unittest.skipUnless(HAS_MULTILINGUAL, "plone.app.multilingual is not installed")
+class TestMultilingualLanguageControlpanel(unittest.TestCase):
+
+    layer = PLONE_RESTAPI_DX_PAM_FUNCTIONAL_TESTING
+
+    def setUp(self):
+        self.app = self.layer["app"]
+        self.portal = self.layer["portal"]
+        self.request = self.layer["request"]
+        self.portal_url = self.portal.absolute_url()
+        setRoles(self.portal, TEST_USER_ID, ["Manager"])
+
+        self.api_session = RelativeSession(self.portal_url, test=self)
+        self.api_session.headers.update({"Accept": "application/json"})
+        self.api_session.auth = (SITE_OWNER_NAME, SITE_OWNER_PASSWORD)
+
+    def tearDown(self):
+        self.api_session.close()
+
+    def test_update_language_does_not_sync_site_language_on_multilingual_site(self):
+        if not IPloneAppMultilingualInstalled.providedBy(self.request):
+            self.skipTest("plone.app.multilingual is not enabled")
+
+        registry = getUtility(IRegistry)
+        language_settings = registry.forInterface(
+            ILanguageSchema, prefix="plone", check=False
+        )
+        old_default_language = language_settings.default_language
+        old_site_language = self.portal.Language()
+        new_language = "de"
+
+        try:
+            self.portal.setLanguage("en")
+            language_settings.default_language = "en"
+            transaction.commit()
+
+            response = self.api_session.patch(
+                "/@controlpanels/language",
+                json={"default_language": new_language},
+            )
+            transaction.begin()
+
+            self.assertEqual(204, response.status_code)
+            self.assertEqual(new_language, language_settings.default_language)
+            self.assertEqual("en", self.portal.Language())
+        finally:
+            language_settings.default_language = old_default_language
+            self.portal.setLanguage(old_site_language)
+            transaction.commit()