What to do with a "letter of happiness" from Germany?

Question

Law in IT

2 years ago I released an application for iOS, a psychological test.
The basis for the test is a bunch of open sources describing the methodology.
Yesterday I received a letter from Apple that there is a request from the copyright holder in Germany.
I contacted them and asked what exactly I had broken. This is a law office.
They sent a claim (in general, probably justified). Although at the time when I did the app, it was impossible to find out that I was violating their rights (well, or I was looking badly, although honestly I was looking).
They demand to pay 5,000 euros within a week, or they will demand 150,000 euros in court.
I think what to do. I am a resident of the Russian Federation.
Can anyone come across this?
What could be the consequences?
How does Apple behave in this case?

Answer 1

Go to legal advice first.
But in general, most likely the wiring.
In the presence of real complaints, Apple blocks applications almost instantly.

Answer 2

Ask them how they justify the amount of 5,000 euros and for what reasons the court requires 150,000 euros, well, send them a counterclaim.

Answer 3

Send nah. Although it is possible that Apple will remove your application from the store. This will be all your losses.