Can any one please find a movie call . one summer on a lake? I think its german

There is a movie that had its German name "Ein Sommer am See", what means "A Summer on a Lake". However, it is a British movie with original title "A Month by a Lake" (1995), http://www.imdb.com/title/tt0113849/

As the movie is not a typical one that someone would look for on RFF, I guess that you had some other movie on your mind, but you should really provide some more information.

The first movie that appeared on my mind with children, a lake and Germany or Austria was "Sommer mit den Burggespenstern", Austrian coproduction with Canada, known as "A Summer With the Ghosts" (2004), http://www.imdb.com/title/tt0388446/

But there are dozens of movies that could fit in your short description.