As counter-argument, I present any American who has managed to learn German well enough to be understood when he/she says anything at all. The sounds represented by ä, ö, and ü don't crop up in English very often (if at all - no examples spring to mind), and if an average American child is exposed to German, it's usually in something like a WWII movie - meaning the speaker is almost certainly shouting, so the sound is probably lost.
On topic... I can't mimic to save my life. I spent 1.5 years outside of London when I was small, and had since an early age a speech impediment that required seven years of speech therapy to make intelligible (r, l, d, s, t, th, sh, ch, w - I still have trouble with things like "world"). I'm Eastern American, generally (born in New England, live in Chicago until 3rd grade, and am now in central PA). If I spend too much time with someone with a London accent (the other English accents have no effect on me, it seems), I seem to revert a bit - especially if I've had a couple drinks. This makes sense, as many of my old problem sounds are what are immediately different between the two accents. Interestingly, if other Americans have been drinking, I suddenly possess a clear London accent, regardless of my intoxication, which is sure to disappear as they sober up. Several of the freshman at my school now believe that I fake a British accent when they're drinking to mess with them.
I prepared Explosive Runes this morning.Alex Keene