The easiest way to add a new language is to import it through the Data Tool. See @10.4. Importing language packs‍ for more information.

If you later upgrade Kartris, you may find that new English language strings were added, but that your other languages lack these records. This can be addressed through the languages page - see @15.1. Languages and email setup‍ 

Once you have 'fixed' the other languages so that they have the same number of strings, you may need to translate those new English strings into the desired language. This can be done from this section. You can choose the target langauges, and filter records to find the strings you need to translate, then enter the new values.