TYPO3 Core

See

http://wiki.typo3.org/TYPO3_4.6 under Backend Development:

Language files: Localization parsing is now cached within typo3temp/Cache/Data/t3lib_l10n. This speeds up TYPO3 but may lead to weird behaviour when you add a new language label in a locallang file and it seems not to "exist" at all. Instead of clearing the aforementioned cache directory, you may add an item in the clear cache menu by adding enabling it in localconf.php:

$GLOBALS['TYPO3_CONF_VARS']['SYS']['lang']['cache']['clear_menu'] = TRUE;

I can confirm that Clear all chaches clears all caches.