I have been working across Europe since 1994 - my UK based company has held contracts with the US and Europe. Most my contracts have reference that state it will be handled in the English courts, one covered Dutch courts, another covered US courts. I was happy with the money, I did not expect to fall out with the client and thus signed, did my thing and took their money.
If you use a contract that says German courts, and you/your client have disagreement then you will have to fight it out in Germany.
While German lawyers can be expensive, unlike the US, many are bound by law to fixed costs pending on the advice/service they are offered (They can ask you to wave this right in order to charge you more). I mention this in such that do not assume that a German lawyer is going to put his kids and grand kids thru school on your invoice.
I would seek advice if not already too late - I was reading into patents recently and believe that sharing a patent prior to approval, can negatively impact your claim to it if there is a dispute later. NDA will offer some protection, but don't assume you are fully covered.
Side note: I belong to a Berlin facebook starter group and someone I never met recently made a post there. They supposedly are US based, part time Berlin based patent lawyers. I am not endorsing them - merely recalling a conversation thread there. Search Facebook Berlin Startups and join/ask there for more info.