TYPO3 Core

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Here is a patch that may help:

--- ../../typo3_src-4.2.1.orig/typo3_src-4.2.1/t3lib/class.t3lib_tceforms.php 2008-06-11 10:14:58.000000000 0200
++ class.t3lib_tceforms.php 2008-09-05 13:57:40.000000000 +0200
@ -2255,6 +2255,7 @ * @return string The HTML code for the TCEform field
*/
function getSingleField_typeFlex($table,$field,$row,&$PA) {
+ global $BE_USER;

// Data Structure:
                $dataStructArray = t3lib_BEfunc::getFlexFormDS($PA['fieldConf']['config'],$row,$table);
@ -2310,7 +2311,7 @

$PA['_noEditDEF'] = FALSE;
                        if ($langChildren || $langDisabled)     {
-                               $rotateLang = array('DEF');
+                               $rotateLang = array(strtoupper($BE_USER->uc['lang']));
                        } else {
                                if (!in_array('DEF',$editData['meta']['currentLangId']))        {
                                        array_unshift($editData['meta']['currentLangId'],'DEF');
@ -2338,7 +2339,7 @
                                                // Render sheet:
                                        if (is_array($dataStruct['ROOT']) && is_array($dataStruct['ROOT']['el']))               {
                                                $lang = 'l'.$lKey;      // Default language, other options are "lUK" or whatever country code (independant of system!!!)
-                                               $PA['_valLang'] = $langChildren && !$langDisabled ? $editData['meta']['currentLangId'] : 'DEF'; // Default language, other options are "lUK" or whatever country c
ode (independant of system!!!)
+                                               $PA['_valLang'] = $langChildren && !$langDisabled ? $editData['meta']['currentLangId'] : array(strtoupper($BE_USER->uc['lang']));       // Default language, other
 options are "lUK" or whatever country code (independant of system!!!)
                                                $PA['_lang'] = $lang;
                                                $PA['_cshFile'] = ((isset($dataStruct['ROOT']['TCEforms']) && isset($dataStruct['ROOT']['TCEforms']['cshFile'])) ? $dataStruct['ROOT']['TCEforms']['cshFile'] : ''
);

Please take care for the rules¹ to create and post a patch

[1] http://typo3.org/teams/core/core-mailinglist-rules/