diff --git a/src/Http/Middleware/CP/Localize.php b/src/Http/Middleware/CP/Localize.php
index 2a3e83c9b15..7f972b90fc3 100644
--- a/src/Http/Middleware/CP/Localize.php
+++ b/src/Http/Middleware/CP/Localize.php
@@ -4,6 +4,7 @@
 
 use Closure;
 use Statamic\Facades\Preference;
+use Statamic\Facades\Site;
 
 class Localize
 {
@@ -11,9 +12,17 @@ public function handle($request, Closure $next)
     {
         $locale = Preference::get('locale') ?? app()->getLocale();
 
+        if (! Preference::get('locale')) {
+            $locale = Site::selected()->locale();
+        }
+
         // Make locale config with dashes backwards compatible, as they should be underscores.
         $locale = str_replace('-', '_', $locale);
 
+        if (str_contains($locale, '_')) {
+            $locale = strstr($locale, '_', true);
+        }
+
         app()->setLocale($locale);
 
         return $next($request);