That’s not true - you can have a contract in any language in Germany. But the first thing that a court adjucating any dispute would do is have a certified translator translate it into German - at your cost. And that translation will be out of your control. So it makes sense to have a German contract, so at least you know what the contract says going to court.
Plus, if during the certification of the translation it comes up that the document does not comply with the legal requirements, it may lead to a contract invalidation. It can be really expensive depending on the matter of the dispute.
Obviously not true for many countries. In Sweden we often use English even for domestic contracts. I've seen employment agreements in English between a Swedish company and a Swedish employee.
You can choose both the law and forum in many types of commercial agreements. Typically international contracts will choose English law as the governing law.
