A polite letter informing them they are in violation of labor laws should be sufficient. Their gamble is you’re afraid to assert your rights, and if you can make clear that you understand they are guaranteed to lose a court case if it gets to that point they’ll pay you swiftly. European companies really don’t want to get a reputation of getting sued by their employees and losing, and they also don’t want to anger local judges.
Make sure your letter(s) are well informed, factual, nonemotional, and don’t make any threats (explicit or implied).
I had the same problem long time ago and I contacted a lawyer. He suggested that I write a letter in which I give the company 2 weeks to transfer the money with the threat of calling a lawyer if it is not transferred. And only then get an lawyer. The company transferred the money and the problem was solved in my case.
Check your contract, it will usually say disputes have to be performed in Germany.
It just doesn't sound like there's enough money at stake to pay german lawyers, manage the case and come out ahead even if they pay.
You can try a place like this (just a result of a quick google, no idea about them) https://www.bierensgroup.com/en-gb/debt-collection-germany/
Getting chased up in german, from germany, in the correct form might panic them into paying.
I’m not in Germany but as it’s in the EU I would contact a German employment lawyer.
Write a short summary of the events, detailing what they should have paid you, when they should pay it, what they did pay you, when they paid it, and how much they still owe you. Also include a copy of your contract.
Use bullet points and make it as easy to read as possible. Imagine you’re asking a stack overflow question.
Probably the lawyer will give you a free initial consultation where they tell you the next free steps.
Maybe the lawyer will charge you a few hundred euros to send a letter before action.
If the company doesn’t pay, and is still solvent then the lawyer might recommend taking them to court as the company will have to pay the lawyers fees if you win the case.
It’s likely your case is open and shut and it’ll never go anywhere near court.
Name and shame ;)
Where are you from ? What's your occupation and nature of contract?
I assume if you're based outside of EU you were paid as a contractor? In that case you can start a legal dunning process (gerichtliches Mahnverfahren), that's actually surprisingly easy to start, most municipalities provide it as an online service. The court will ask you to provide some proof of the unpaid services you provided and send a court order to the company, most companies crater after that and pay. Once you have an enforcement order (Vollstreckungstitel) you can start the debt enforcement process (Zwangsvollstreckung), an official court agent will seek to retrieve the debt from the company then, going as far as seizing office assets and even cash in the founders pockets (it can be quite humiliating). As I said, it's a surprisingly easy process to get started and it's really intimidating to most companies as any legal action might lead to being publicly documented and tainting the credit score of the company, it can even lead to court ordered bankruptcy filing.
If you were actually employed you need to take the company to the labour court (Lohnklage), given how difficult it is to employ someone who's residing outside of the EU I imagine this to be even scarier for the company, so they should be very willing to settle their debt with you.
Again, the key point is to plausibly threaten and possibly enact these measures. This is not legal advice.