For now, Overdub is only available in English.
Overdub training sessions are special Descript projects associated with custom Overdub voices. They contain the training media used to create the voice and must be submitted for training and verification. You can access training sessions by selecting a row for the voice in the Voice tab of your Drive view.
They work like any other project in Descript; the only difference is that you cannot publish or export a training project. Instead, you will see the Instructions and Submit for training buttons in the top right corner of the app.