Hi @waszczuklukasz,
You can add a image going to your screen settings!
If you need that to be Dynamic you can use this component made by James! : 🎉 It's HERE! Dynamic Background Component NOW available in the Component Marketplace!
Currently there is no direct way to do this, unfortunately 
Vote here : Support for Multiple Languages | Voters | Adalo
Here’s a one way by Pragmaflow using a component that Pragmaflow built : Multilingual App
Here’s a second way by Yongki : Free Template - Komun for Community Support - In Progress - #6 by Yongki
Thank you