Now the fun part: It just takes some seconds to add new languages. Head over to your customizer tool (https://app.translatestack.com/customizer) and click the Languages tab. You'll find a drop down, where you see your current target languages. On the right, you'll see an imitates version, of how your drop down looks in production.
Simply search for your language you want to add a press "Save".
Once you hit the save button, your new language will appear in your dashboard and you are able to add all translations to it. Simple red indicators help you to spot all strings where translations need to be added. Additionally, a "translated" value helps you understand how many percent you have already translated.