In one of his Austin Powers movies, Canadian actor Mike Meyers portrays speaking English with a Dutch accent as basically having some sort of lisp. If you really want to hear what a Dutch accent sounds like, listen to British comedian Bill Baley when he is introducing human beatboxer Shlomo. One minute into the following Youtube clip he compares his own beatboxing skills to those of a “burnt out Dutch stockbroker who is into a bit of Dutch hiphop”. His vowels are particularly good — and he pronounces them while beatboxing!
When I try to speak English, its my vowels that betray my Dutchiness. Plus, of course, the fact that I use words like “Dutchiness”.
(Oh, and “eftenfiften” is not a Dutch word, nor is “achtung”.)