In your store's Admin area, go to the System > Localization > Languages
The most convenient way to add new language is Install language extension. You can find few already added with cart installation in Admin > Extensions > Languages or download and install new.
Check for language extensions at AbanteCart marketplace and about 25+ languages available at github
Check list with about 25+ language extensions we create for you using machine translation. Please help the AbanteCart open source community to correct (translate) language extensions xml files. Translating an open source project is much like the development of the project itself.