Get Translation on Moodle using "get_string" for a specific language

Question

Translate on Moodle using "get_string" for a specific language.I am trying to get a translation of my plugin block on Moodle in a specific language. but I cannot. here is a code example:

    public static function course_lesson_created(\mod_lesson\event\page_created $event)
    {
        global $DB;

        $event_data = $event->get_data();
        // Get the course ID from the event data

        if (isset($event_data['target']) && $event_data['target'] == "page") {// if lesson page
            $objectid = $event_data['objectid'];

            // Query the database to get the lesson page
            $lessonPage = $DB->get_record('lesson_pages', ['id' => $objectid], '*', MUST_EXIST);
            $lesson = $DB->get_record('lesson', ['id' => $lessonPage->lessonid], '*', MUST_EXIST);
            $course_module = $DB->get_record('course_modules', ['id' => $event_data['contextinstanceid']], '*', MUST_EXIST);


            $response = self::create_MoodleContentDetail(
                $lessonPage->title,
                $lesson->intro,
                $event_data["action"],
                "lesson",
                "<h1>" . get_string('lessontitle', 'block_smartteacher', null, $course_module->lang) . ": {$lessonPage->title}</h1>" .
                "<h2>" . get_string('lessondescription', 'block_smartteacher', null, $course_module->lang) . ": {$lesson->intro}</h2>" .
                "<h3>" . get_string('lessoncontent', 'block_smartteacher', null, $course_module->lang) . ": {$lessonPage->contents}</h3>",
                $course_module->lang,
                $course_module->course, // courseId
                $course_module->module, // moduleTypeId
                $course_module->id, // moduleId
                $course_module->section  // sectionId
            );


            //  var_dump(json_encode($event_data));
            //  var_dump(json_encode($response));
            /*      var_dump(get_string('lessoncontent', 'block_smartteacher', null, $course_module->lang));
                 var_dump(get_string('lessoncontent', 'block_smartteacher', null, "ar"));
                 var_dump(get_string('lessoncontent', 'block_smartteacher', null, "en")); */
            // var_dump($course_module->lang == "ar");
            // var_dump($course_module->lang == "en");
            //  var_dump($course_module->lang == "ar" ? "ar" : "en");
            //  die();
        }

    }

Is my way wrong get_string('lessontitle', 'block_smartteacher', null, $course_module->lang) ? I got that lang is correct its return "ar"