Being database-driven, the Kartris language provider is very efficient. However, to improve performance even more, language strings are cached because in most cases they change very rarely. If you change a language string, you may find that the change does not appear immediately, even if you rebuild the caches with the button above the left hand category menu. In this case, you can force the language strings to be rebuilt by restarting the site. This can be done from DB admin.