I've been pondering this question. As my German language skills have improved, I've been having longer discussions in German. Not debating philosophy, of course. They haven't progressed that far!
But I have noticed a couple of things about German which I believe impacts the culture. Or vice versa.
1. It is difficult to form German words and smile at the same time. Just the way you have to shape your lips when speaking makes a smile feel unnatural.
2. Because German sentences tend to be quite long and the action verb generally doesn't appear until the very end, everything you intend to say has to be carefully considered and planned. Compound this with the very structured word order and spontaneity becomes extremely difficult.
So, did the Germanic culture produce a language that encourages formality and discourages smiling? Or did the German language produce the stoic culture? Or do they just build upon one another?