Except for beginning level students of English, I always encourage (and sometimes politely force) my students to use the English-English dictionary. If students don't check the English-English dictionary, they won't fully understand the nuance of the word. Additionally, the English-English dictionary usually has a few examples to get a further feel for the word.
Of course, a dictionary in the students' mother tongue can be looked at, especially if they aren't quite sure of the definition provided in English. But this would only be a tool to support the English-English dictionary, not a substitute.
I've found that most translation dictionaries just throw a bunch of words onto the page, so there isn't an ability to understand just which word is correct for which situation. I've also found that by consulting an English-English dictionary, students are more able to explain around unknown words and listen to and understand explanations.
Chris Cotter
Heads Up English - Just print, and teach!
English Lesson Plans | Heads Up English | ESL EFL