Pigeon

There is a polish application called "IVONA", which is a complex voice-synthesizer for over 50 languages from all over the world. With enough knowledge of voice-editing the results may be satisfactory

Here you got polish installer for your setup polish_translation.7z

From my personal experience (polish translation) i may tell you, that far way easier it would be to add voice actors, because all You would have to do, is to mute vocals and add new ones. Problem is to find any volunteers for this