On desktop, you can switch your interface by editing the browser URL. Below is an example demonstrating how to switch from a German interface to an English interface.

Before:

https://www.lingq.com/de/learn/nl/web/feed

After:

https://www.lingq.com/en/learn/nl/web/feed
 
The mobile app interface is a system preference, so your LingQ mobile app will be in whatever language your phone interface is in.