Change language Skype for Business WITHOUT o365

Hi, 

I'm currently trying to change the display language for one client from German to English. However I don't see any option to do so. We're not using o365. And there aren't any options in the Skype for Business installer.

We have Skype for Business 2015 which is completely separate from our Office 2016 installation. So changing Outlook to English doesn't change Skype for Business.

How can we change the Skype for Business client display language?