After installing this script and editing your menu.ini file you can get google translate translations in a small pop-up dialog, so you don't get taken to a different page, as with Opera's default translation feature.
What it looks like
(click for full size picture)
Setting it up
After installing the the script as per the instructions on the right, close Opera and open your menu.ini file, it's a good idea to make a duplicate first and work on that. At the bottom of the page add.
Adds an insert special characters toolbar (e.g. accented) to Google Translate.
Adds a row of buttons to the top of the Google Translate textarea which insert the indicated character at the current caret position. Hold the shift key to make the characters upper case. Currently supported languages:
