[3.0] Ensure we create en_US when copying the theme

Fixes #8974

Author: jdarwood007

Sources/Actions/Admin/Themes.php

@@ -1478,6 +1478,7 @@ protected function installCopy(): array
 
 		// Create subdirectory for language files
 		mkdir(Utils::$context['to_install']['theme_dir'] . '/languages', 0777);
+		mkdir(Utils::$context['to_install']['theme_dir'] . '/languages/en_US', 0777);
 
 		// Copy over the default non-theme files.
 		$to_copy = [