Core

There was a hook in t3lib_div::readLLFile() to override any locallang file by another one. The additional locallang is merged with the existing one which is very nice feature to override just one label.

as the l10n changed a lot with xliff, it is not just a copy paste to get hook working again.
previous hook was this

$overrides = array();
            $fileRefWithoutExtension = preg_replace('/\.(php|xml)$/', '', $fileRef);

            if (is_array($GLOBALS['TYPO3_CONF_VARS']['SYS']['locallangXMLOverride'][$fileRefWithoutExtension . '.php'])) {
                $overrides = array_merge($overrides, $GLOBALS['TYPO3_CONF_VARS']['SYS']['locallangXMLOverride'][$fileRefWithoutExtension . '.php']);
            }
            if (is_array($GLOBALS['TYPO3_CONF_VARS']['SYS']['locallangXMLOverride'][$fileRefWithoutExtension . '.xml'])) {
                $overrides = array_merge($overrides, $GLOBALS['TYPO3_CONF_VARS']['SYS']['locallangXMLOverride'][$fileRefWithoutExtension . '.xml']);
            }

            if (count($overrides) > 0) {
                foreach ($overrides as $overrideFile) {
                    $languageOverrideFileName = self::getFileAbsFileName($overrideFile);
                    if (@is_file($languageOverrideFileName)) {
                        $languageOverrideArray = self::readLLXMLfile($languageOverrideFileName, $langKey, $charset);
                        $LOCAL_LANG = self::array_merge_recursive_overrule($LOCAL_LANG, $languageOverrideArray);
                    }
                }
            }