If you have any questions, please post on the Internationalisation and localisation forum.
|See more in the Administrator's guide.: using CiviCRM in your own language|
Under the hood, CiviCRM uses gettext to localize the text, and since there are a lot of strings in CiviCRM, the translations themselves are managed in a community called Transifex. is an online community of translators. In many ways, it is the "github" of translation. You can use the web interface to join a translation team (or create a new one) and translate the strings or improve existing translations from there. Extensions can be translated as well, and follow the same principles.