Well, an uneducated Korean impression of american english goes something like this:
“err arr shar shwar ir,”
Basically a random combination of the’ r’ sound with the ‘sh’ sound.
I think people woud tend to imitate the part of the language that is the most alien to them. For example, the english ‘r’ sound is rarely found in Korean and the ‘sh’ sound is usually only found in conjunction with the ‘ee’ sound (like in “shee bal”, which you might hear a korean say quite often).