20:10 Apr 19, 2011 

it is a matter of what looks good and sounds good in English. And there may be that some of those lakes have established names on authoritative maps that might be worth adopting.

11:59 Apr 19, 2011 

it is quite tricky..
I would agree with your own suggestions, i.e. Lake Kummerow, Kölpin Lake (proper names always with upper case)
As for "Plauer See"we need to know that it is a lake in Mecklenburg/Vorpommern near the town of "Plau" so the "-er" ending has to be left out ; hence Plau Lake (really it is probably just a matter of what is easier to say Lake Plau or Plau Lake; (upper case in either case :) I am not sure whether there is necessarily a rule for it.
Of course, Bodensee becomes "Lake Constance", so go figure.
I am not sure if this isd helpful, but your hunch was right. The translation had not been done consistently.