'Translate' item in select + right-click > Actions...
When a user selects a string in the chat widgets, then right-clicks on it, the 'Action' contextual menu appears, showing different items including 'Search on the web'.
Gajim should include a 'Translate' item, redirecting to the translate page in the default brower, when the user only has to select 'from' and 'to' languages.
Do not forget the &sourceid=gajim at the end of the URL