I downloaded the app in the UK and a English voice.
I have gone on a business trip to France and waze decided that because I was in France I must want a French voice, automatically downloaded it (while roaming!) and switched. When I needed voice directions in a city centre and turned on all sounds I was given directions in a French voice. What was even worse was that when I switched back to English, it had to download the voice again.
This is pretty bad and needs to be fixed. It should not download or switch just because you're in a different country.