Update: it works now with the latest version of Intellij IDEA 7 (Selena; build 7118).
How to use it in side IDEA editor:
1. In editor select the part to be translated;
2. Select "Translate" item from the editor popup menu;
3. (hidden) "translate.google.com" web site (service) will perform actual translation;
4. Response from the service will be inserted in place of your selection.