Hi Susanne:

well just trying to shed some light here as i was living at the most Frissisch place in Germany, called Wittmund. The first verse has sofar been translated very well.

the second verse, well someone misstook Dünen(dunes) and Dänen(Danes). this part talks about the Danes walking along the shore from Greenland down to Belgium. Which they did a few centuries back while traveling, the easiest way was to go from the north sea, also crossing the Netherlands.

And the third verse is as good a guess as any about the Seals(which if you think bout them always being on their bellies, could make you think as to why they can´t fan themselves with their tales, or move their "Schwanz" around.
While the last verse talks bout sheep gazing stupidly and leaving their droppings around, there is also a game i think it´s called Bossel, or something that sound very similar, which is only known in that part of germany, which consists of throwing around the dikes and brush, balls of various colors, but most noticeably(pardon my grammar), black and green ones, in order to make the harder by being unable to find them once you loose your ball. so they might be talking about the same Frissian people who decorate the dikes with their green/black balls while staring stupidly for them.

hope this was of some help at all, and quite funny to remember such songs