OK, there are problems with with prompting bookmarklet in IE. It's all to do with the prompt. Yesterday it worked, but only on the first use, then it would stop prompting for the language-pair value (and in fact ignore the default value I'd put in there) and just skip straight to the translation page, which, without a language pair, can't do much. I think it may be to do with security, since it occasionally shows that "website trying to show active content" warning for a moment before scooting straight off to Google without a by-your-leave, let alone that prompt.
So for IE, for now, I'm just using a straightforward Italian-English bookmarklet
For Mozilla, the prompting version is now also Google-based and goes there. Here it is: translate