There is a language button changer on the official website, but it appears that it's not working. However, I've extended the investigation further it and appears that English isn't a supported language. All the screenshots are presented in German language. Also on the Windows Store, the official language is displayed as Deutsch (Deutschland).
You can forward an email to the developer to ask them about the possibility to use the Skat-As in English language. Access the following page and if you can't understand what it says, right click on page and choose Translate to English.