“We Americans overpromise. Much more than do our German colleagues. How do we strike a balance between overpromising to our American team-leads – and/or to our American customers – and underpromising or realistic-promising to our German colleagues?”
Reduce the overpromising to your American team-lead. Get real. Get realistic. Only promise what you can deliver. Reduce inflation in the broadest sense in the U.S. Take that chapter from the German book. And encourage your American team-lead to reduce their overpromising to their next level management.
On the other side of the coin, encourage your German colleagues to aim higher. The term encourage means literally to give courage. The Germans are chronic underpromisers, to a fault. The can reach higher and achieve more. They take things too safely, too often.
Third, make this a topic in your collaboration with both Americans and Germans. Stating it in an oversimplified, but accurate, way: Americans are inflationary. Germans are deflationary. Work together towards the middle.
There is no hiding from this cultural difference. Therefore, address it head-on.