The "T" is pronounced like in butter and the "me" is pronounced like in "mess". "Sch" is like in the name Schmitt, that also does exist in the USA.
click on the little symbol of a speaker on both languages:
http://translate.google.de/#en|de|butterfly
I honestly think, that butterfly and Schmetterling don't sound too different, if you don't have any knowledge of both languages. Just close your eyes and compare the acoustic pattern how it may sound for some Vietnamese guy or from the perspective on any language that isn't Germanic or Roman.
Maybe that is, because the number of syllables match.