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Flaak exists in Germany as you can see at http://www.verwandt.de/karten/absolut/flaak.html. It would seem to be a variant of Flack/Flacke which, according to Bahlow, is "from a dwelling place by stagnant, swampy water (in Dutch vlak still = swampy".
You'll find confirmation of a locative origin for the second name at http://tr.im/Gq4W.