The project consists in developing a Jitsi plugin that would have to scan outgoing messages for spelling errors the same way that most word processors like OpenOffice and Microsoft Word handle this. Once an error is detected, the user should be alerted (e.g. by underlining the word) and a right click on the wrong word should should open suggestions with the right spelling.
You would have to find existing dictionaries for as many languages as possible and a common java API that uses them. You may have a look at the NetBeans IDE that includes such a plugin written in java (see the references).
Spellchecker Module for NetBeans
The NetBeans IDE
Other Jitsi GSoC Projects
Jitsi Developer Documentation
The official Jitsi website