Core

With locallangXMLOverride you are able to override any label. A lot of backend labels are still configured like this:

'title' => 'LLL:EXT:lang/locallang_tca.php:pages',

To override such a label, you have to use
$GLOBALS['TYPO3_CONF_VARS']['SYS']['locallangXMLOverride']['EXT:lang/locallang_tca.php'][] = 'PATH_TO_MY_XML';
whereas the label is stored in an locallang_tca.xml file.

readLLfile removes the file extension when it is called with a filepath, checks if there is a xml file and uses that one. If there is no xml, it checks for a php file.

When you use locallangXMLOverride however, you need to set the filepath not to the actual path of the xml file, but to the path with which readLLfile is called. This is inconsistent and so you have to include two override statements to make sure to catch all labels.

Solution: check if there are overrides set for php and xml paths

Note: patch is for current trunk, but applies to 4.4 with some offset

(issue imported from #M15942)