What steps can I take in Germany to sue a supplier for breach of a long-term contract and recover damages?

It can help to try mediation or arbitration first if there is a chance for an amicable solution, especially given the lengthy collaboration. The legal costs depend on the amount of damages or repayment claims in question. How much do you intend to ask? Court cases usually take 1 to 3 years. Chances of success of course depend on the legal situation, which requires much more information. If you have further questions, send me an email to hi(at)streiff.law