Hq location can be quite irrelevant. Legal intercept laws can be quite old-fashioned and might make a case than two German citizens having a conversation while on German soil makes the conversation fall under German jurisdiction. There can be a surprisingly large number of ways the jurisdiction can be determined, for all parties involved and, without analysis of German law, I would not readily make assumptions as to if they have a legal basis to talk to Signal or not. And if they do, I’m sure Signal is a law-abiding company.