If your language is missing at all, then you need to click the link "Request language". The maintaners will then approve (or not ) your request and add that language. If I remember right, you should be part of that language team then already, as you requested it. If not - see below. If you want to work on an existing language, then you need to click on the language link you want to translate and then click "join Team" link. After you got approved by tgalal, berdario or cepiperez you can start translating that language.