"German police have to find the wallet he lost in Germany."
Translation:Almanya'da kaybettiği cüzdanı Alman polisi bulmak zorunda.
Kaybettiğin has (because of the -dik suffix) become a modifier.
The modifier precedes the modified; they have to stick together as a word group.
The phrase kaybettiğin alman polis means the German police that you lost.
Almanya'da cüzdanı kaybettiğin alman polis bulmak lazım -> In Germany, the wallet has to find the German police that you lost.
Standard word order in Turkish is subject - object - verb:
Subject: German police
Object: the wallet he lost in Germany
Verb: have to find
I suppose that the fact that subject and object are in reversed order means that Duo wants more emphasis on the idea that it's specifically the German police that has to find the wallet.
I cannot see in which lesson this sentence is included but I try the -dik lesson. Please write again if it's another lesson I will try to help you.
Really understand it's a pity you can't get the grammar on the phone.