Go to the project page, which you want to translate (see above)
a) If your language team already exists, click on it and request to become a member of it.
In this case either the project management and the language team coordinator of that language can accept or deny the request.
b) If your language team does not exist, you can request to create a new one and become a language team coordinator.
In this case the project management has to accept or deny the request.
The language teams are organized within https://www.transifex.com/projects/p/MediaPortal/
So currently it's Albert, morpheus, offbyone and Chefkoch who are able to accept new languages.
In most cases we react within 24 hours on request, often even earlier.