Start deprecation for the locales as a template tag

- This was only used by our JS config globals and can now be removed in a future release

docs/releases/6.3.md

@@ -91,3 +91,9 @@ To disable the automatic preview update feature, set [`WAGTAIL_AUTO_UPDATE_PREVI
 ## Upgrade considerations - changes affecting Wagtail customisations
 
 ## Upgrade considerations - changes to undocumented internals
+
+### Deprecation of the `{% locales %}` template tag
+
+The undocumented `locales` template tag will be removed in a future release.
+
+If access to JSON locales within JavaScript is needed, use `window.wagtailConfig.LOCALES` instead.

wagtail/admin/templatetags/wagtailadmin_tags.py

@@ -867,7 +867,15 @@ def locales(serialize=True):
         }
         for locale in Locale.objects.all()
     ]
-    return json.dumps(result) if serialize else result
+
+    if serialize:
+        warn(
+            "The `locales` template tag will be removed in a future release.",
+            category=RemovedInWagtail70Warning,
+        )
+        return json.dumps(result)
+
+    return result
 
 
 @register.simple_tag