What does "buttermilk" mean in an American pancake recipe?
You are correct that in the US buttermilk refers to cultured milk and not the soured leftovers from making butter. Historically buttermilk was the liquid left after making butter that had fermented during the accumulation of the cream. It was described as milky and sour- not creamy like modern buttermilk. 

Your recipe is certainly referring to the cultured variety. I have never seen or heard of actual buttermilk being sold anywhere and so I have no idea if it can be used in the same recipes.

The milk doesn't have to be whole. The product is thick, creamy, and tart not unlike a loose yogurt.

I don't know German but I can say that Kefir (if it is the same product as one with the same name in Russian) is definitely not buttermilk. It can be used as a substitute as it has a similar texture but the flavor is very different.