I managed to solve 9 of the puzzles in the lake before we visited in 2011. Fanto, who has set many of them, speaks English and was helpful in verifying my thinking and answers (there don't seem to be many with geocheckers). The ones I solved were all on the mainland rather than on the island. How's your German? In the event, we didn't have time to visit any of the puzzles and just settled for a few trads on the island while walking around the town.
Jack
fitzsimons