the voice has nothing to do with the language. i think it would require that the directions/voice prompts would have to be passed through a translation engine, and then synthesized using flite.

not sure, but i think gnuite was doing something with mapper so that one could pick his/her locale and have the gpx downloaded in the native language. not sure if that was implemented or even on the road map, but i heard whispers to that effect some time ago.

if it was the case that one could download routes, with directions in their native language, the flite should be indifferent, and be able to give the directions in the language specified. i think the pronunciation may be a bit off, and that would make for some fun listening.