Can buttermilk be frozen successfully

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.

Hah, I get to cite my new copy of McGee's "On Food and Cooking" for the first time! There are several things going on here (all of which can be found in the 2004 edition of McGee, most on page 50).

Firstly, as Nathan indicates in his answer, most of the liquid that is sold as buttermilk these days is in fact not "real" buttermilk, but so-called "cultured buttermilk", made from ordinary skim milk and fermented until acid and thick.

Secondly, even traditional buttermilk was somewhat sour, though less than cultured buttermilk. To find out why, let's examine the processes that were involved in making it. If you want to make butter, you start by separating milk into cream and whey. Since the 19th century, we typically do that with a centrifuge and it goes quite quickly, but before then, we would leave it to gravity. That would take a while, and the milk would start to ferment while it happened - especially in warmer environments. This fermenting is essentially lactic acid bacteria converting lactose into lactic acid, which sours the milk a little.

So in either case, you would have more lactic acid in butter + buttermilk than in butter + whole milk.