fix: IncomingRequest setLocale not falling back to generic language code

system/HTTP/IncomingRequest.php

@@ -441,10 +441,14 @@ public function getPath(): string
      */
     public function setLocale(string $locale)
     {
-        // If it's not a valid locale, set it
-        // to the default locale for the site.
+        // Check if valid locale.
         if (! in_array($locale, $this->validLocales, true)) {
-            $locale = $this->defaultLocale;
+            // If not valid, check if language only is valid locale.
+            $locale = strtok($locale, '-');
+            if (! in_array($locale, $this->validLocales, true)) {
+                // If neither valid, use default locale for the site.
+                $locale = $this->defaultLocale;
+            }
         }
 
         $this->locale = $locale;

tests/system/HTTP/IncomingRequestTest.php

@@ -223,6 +223,19 @@ public function testSetBadLocale(): void
         $this->assertSame('es', $request->getLocale());
     }
 
+    public function testSetBroadLocale(): void
+    {
+        $config                   = new App();
+        $config->supportedLocales = ['en', 'fr'];
+        $config->defaultLocale    = 'en';
+        $config->baseURL          = 'http://example.com/';
+
+        $request = $this->createRequest($config);
+
+        $request->setLocale('fr-FR');
+        $this->assertSame('fr', $request->getLocale());
+    }
+
     public function testSetValidLocales(): void
     {
         $config                   = new App();

user_guide_src/source/changelogs/v4.6.5.rst

@@ -30,6 +30,8 @@ Deprecations
 Bugs Fixed
 **********
 
+- **Localization:** Fixed a bug in ``IncomingRequest::setLocale()`` that prevented falling back to valid, supported generic language code as locale.
+
 See the repo's
 `CHANGELOG.md <https://github.com/codeigniter4/CodeIgniter4/blob/develop/CHANGELOG.md>`_
 for a complete list of bugs fixed.