Under Community Review

In fact, it is possible to move tasks from one project to another which could meet this requirement, but indeed it would be more intuitive to have a Edit Project Settings feature that allowed adding (or deleting) a language to an existing Project.

Add new locales to an existing project

It's impossible to add new locales after creating the project. The user needs to cancel the one created and create a new project which is really time-consuming and not handy at all (on both TranPort and WorldServer user interfaces).