[BUGFIX] Only call getModTSconfig in backend mode

Fixes: https://forge.typo3.org/issues/64907

class.tinymce.php

@@ -76,9 +76,11 @@ class tinyMCE {
 		$languageInstance = (TYPO3_MODE === 'FE' ? $GLOBALS['TSFE'] : $GLOBALS['LANG']);
 		$languageKey = $languageInstance->lang;
 
-		$groupOrUserProps = t3lib_BEfunc::getModTSconfig('', 'tx_tinyMCE');
-		if (trim($groupOrUserProps['properties']['prefLang']) !== '') {
-			$languageKey = $groupOrUserProps['properties']['prefLang'];
+		if (TYPO3_MODE === 'BE') {
+			$groupOrUserProps = t3lib_BEfunc::getModTSconfig('', 'tx_tinyMCE');
+			if (trim($groupOrUserProps['properties']['prefLang']) !== '') {
+				$languageKey = $groupOrUserProps['properties']['prefLang'];
+			}
 		}
 
 		// language conversion from TLD to iso631