What does "buttermilk" mean in an American pancake recipe? I want to experiment making American style pancakes (I usually make Austrian style unleavened palatschinken). The recipe I found says "buttermilk", but I remember reading somewhere that the Americans sometimes misuse the word for milk thickened by fermentation. What should I use for the recipe, real buttermilk or fermented whole milk? 

And if you have ever shopped in a German supermarket, do you know which product would come the closest to what the recipe author meant? I have the choice between buttermilch, sauermilch, dickmilch and kefir. Currently leaning towards sauermilch (it isn't really very sour, tastes actually less sour than yogurt). 

Note I am not asking for substitutions trying to approximate acidity by curdling milk with different acids available in the kitchen. I know of this method and don't intend to use it. This question is asking which existing milk product is closest to the one used in the original 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.