[FEATURE] Enable subject to be localizable

If subject is a string that begins with LLL:, it will be handled
as a locallang reference and translated accordingly

Classes/Controller/MailController.php

@@ -38,6 +38,7 @@ use TYPO3\CMS\Core\Utility\VersionNumberUtility;
 use TYPO3\CMS\Extbase\Mvc\Controller\ActionController;
 use TYPO3\CMS\Extbase\Object\ObjectManager;
 use TYPO3\CMS\Extbase\Utility\LocalizationUtility;
+use TYPO3\CMS\Lang\LanguageService;
 
 /**
  * Controller for the mail templating service module
@@ -157,8 +158,18 @@ class MailController extends ActionController {
 				$templateFromFile = new Template();
 				$templateFromFile->setLanguage($key);
 				$templates[$key] = $templateFromFile;
-
+				if (\strpos($subject, 'LLL:') === 0) {
+					/**
+					 * @var $languageService LanguageService
+					 */
+					$languageService = GeneralUtility::makeInstance(LanguageService::class);
+					// initialize the language service with the templates language
+					$languageService->init($key);
+					$subject = $languageService->sL($subject);
+					unset($languageService);
+				}
 				$templateFromFile->setSubject($subject);
+				$subject = $registerArray[$parameters['selectedExtension']][$parameters['selectedTemplate']]['subject'];
 				if (file_exists($defaultTemplateFile)) {
 					$templateFromFile->setContent(file_get_contents($defaultTemplateFile));
 				} else {
@@ -224,7 +235,7 @@ class MailController extends ActionController {
 		$this->view->assign('beUserMail', $GLOBALS['BE_USER']->user['email']);
 
 		// get the default language label and pass it to the view
-		$languageService = $GLOBALS['LANG'];
+		$languageService = $this->getLanguageService();
 		$pageTsConfig = BackendUtility::getPagesTSconfig($pageUid);
 		if (VersionNumberUtility::convertVersionNumberToInteger(TYPO3_version) >= 8000000) {
 			$defaultLanguageLabel = $languageService->sL(
@@ -358,4 +369,13 @@ class MailController extends ActionController {
 		$arguments = $this->request->getArguments();
 		$this->redirect('index', NULL, NULL, $arguments);
 	}
+
+	/**
+	 * Get the LanguageService
+	 *
+	 * @return LanguageService
+	 */
+	protected function getLanguageService(): LanguageService {
+		return $GLOBALS['LANG'];
+	}
 }