[ticket/13003] Add missing language keys

https://tracker.phpbb.com/browse/PHPBB3-13003
I'm my first intention, i tought the same things.

But, these three language keys refer to the word "extensions".
With latin language (and maybe others), the conjugated forms of "enabled" and "disabled" are not the same if the associated word is male or female and if it is plural or singular.

That's why I created new lang keys.