If you save, like David asks, it is still not clear whether get an error during save, whether you get no error, but when you re-open customized language strings, your changes are not there, or whether you do not see the changed strings in the browser, when using Moodle.
If the latter, you browser is most likely at fault. Otherwise, you need to turn on debugging and check server logs. It is possible that something is not quite right with your installation; for example, Moodle can't write the language files.
Languages
Language customisation not saving
This discussion has been locked so you can no longer reply to it.