In fact the text that is displayed "someone else is editing this translation..." does not mean exactly what it says.
"is editing" should replaced by "has locked".
All actions done on a given translation (add a new target language, delete a target language, edit, translate, rate (accept/reject), result in a lock of this translation.
Moreover, the lock is not managed language by language. For example, somebody ask for a translation from english to greek, french and german. If somebody start translating in french, then all the translations are locked. That means greek, and german translations can't be done at this time. You must wait the french translation is completed to do the greek translation. I know it does not sound logic but the lock system can't be managed language by language for now. Maybe one day we need an improvement on this.