Sven,
It might be a good idea to incorporate some IVR (Interactive Voice Response) tactics in the design of your Nav. Mainly, allowing the user to record his/her own prompts. This works for the majority of your grammar (North, South, 10 Miles, etc), and allows for dynamic data like street names. We've been doing this in the IVR industry for decades. If you want, email me off-list and I can give you some ideas.
Jason