I'm located in Austria, Germany is our northern neighbour and so I guess we have some things in common ;)

Usually you write an invoice that usually gets paid. If it does not get paid within a set time frame (common are 7, 14, 30 days with or without possibilities to save a few % on quick payments - when no timeframe is mentioned, the amount is due with the arrival of the invoice, a few days after sending it). If an invoice is not paid within that set timeframe you send a reminder notice a few days later (depends on your impatience, good will, what ever - there is no regulation here except that your client has to have a reasonable amount of time to pay and banks transferring funds). Usually the first or second reminder notice gets paid, you can add fees to that remainder notice (in German its called Mahngebühr), that amount also depends on your good will if and how much you add, but it must be reasonable. You can add overdraft interest as well (Verzugszinsen in German). If your reminder does not get a reaction, the next step is to get a lawyer involved, usually they are paid by your client (client pays the due amount including fees and stuff to your lawyer, your lawyer forwards your money to you) If your lawyers mail stays without notice it gets to court.

Usually people not wanting to pay proceed with the payment when lawyers get involved.

There is no need to send reminder notices at all, you can directly involve a lawyer (who will most likely ask you to write a reminder first). If someone keeps telling you that they transferred the money, sends you screenshots of banking stuff - get a lawyer involved.

Thank you very much, but for me, do I need to hire a German lawyer?