Add locale param to the lessons endpoint (filterable)

Author: aristath

classes/class-lessons.php

@@ -58,6 +58,7 @@ public function get_remote_api_items() {
 			[
 				'site'        => \get_site_url(),
 				'license_key' => \progress_planner()->get_license_key(),
+				'locale'      => apply_filters( 'prpl_lesson_locale', \get_locale() ),
 			],
 			$url
 		);