http://terpconnect.u...Chang_ICPhS17.pdf
http://en.wikipedia....Great_Vowel_Shift
A good speech coach can take an American with no foreign language experience and, just by rotating the pronunciation of the vowels, give them quite passable foreign accents in an amazingly short time.
I was in a class where we all picked up French accents in about an hour. I'm sure a native would have found it odd, but other Americans -- our target audience -- would have thought we spoke English as a second language. German was harder, because there are a lot of glottal stops we're not used to.