For a couple of months now, I've been trying to create a list of good reasons why students should use English-only dictionaries. English teacher Alex case has done a better job than I have, and he's included it on the UsingEnglish.com website. If this list doesn't convince you, nothing will!
If you're worried about the price of an English-only dictionary, don't be. I just got a paperback copy of the Merriam-Webster's Dictionary and Thesaurus (http://www.m-w.com) through Amazon.com for the equivalent of about €15 (including shipping).