The Google Toolbar for Firefox does this (and doesn't cause any reflow since it uses a tool-tip). My wife uses it to help her learn English. I don't use it, so

The Google Toolbar for Firefox does this (and doesn't cause any reflow since it uses a tool-tip). My wife uses it to help her learn English. I don't use it, so I don't remember if the following is true, but I would guess that you can configure it to translate between any pair of languages supported by Google Translate.