This is already documented in forum posts, but the more ways
there are to find this kind of information, the better... took me a
while to find. To the point:
You may have experienced that when you change something in one
of the NN.xml files under /umbraco/config/lang (such as a custom
section name), nothing happens. Nothing at all.
The cause:
The translated texts are cached in javascript snippets by the
client dependency system, and never (to my knowledge)
refreshed.
The fix:
1. Delete the folder /App_Data/TEMP/ClientDependency (if there
is one in App_Data as well, kill that as well - that's the location
in older versions).
2. Touch your web.config (or use whatever favourite method you
have to restart the application).
The folder and its contents will now be recreated (and up to
date).
EDIT: For a more permanent fix on a development server - see
Eran's comment below. Use my fix after deploying new language files
to produktion servers.