You need to add the Translations app and choose your language.
Looking at the page for the Translations app it's not clear what you need to do to get your language added, but it does state:
Add the Translations Application to translate, review and vote on translations in your language.
So the first step would be to add the application and see what options are available to you.
I can't comment further as my language - English - is fully translated:
The Translation Application is currently closed for Facebook in English (UK). Please check back later.