This page describes how users can contribute translations to other languages. The process is very straightforward and based on the easy to use Qt Linguist tool which is a part of the Qt toolkit.
Qt Linguist tool which is part of the Qt 4 SDK
Update existing translation
You should regularly check for changes using Subversion (trunk/src/translations) or by visiting the ViewVC website.
After translating all strings upload the *.ts file to the translations forum.