I found online a solution. Like to share it here.
Disclaimer, it is your own responsibility to use thissolution and I am not sure if it may be used by the Google Terms of Services. I also don't know and did'nt test it, if it will harm other parts of the Android platform, software on your phone and the Google Navigation software for now and/or future.
End of the disclaimer. I like android by it opensource status. It is easy to change files and so on.
Explanation of the hack:
1. Google navigation uses TTS (Text to Speech) by the Pico engine. If you use another TTS engine then Pico then Google Navigation has a fallback to its own soundfile. Because we use the own soundfile of Google Navigation we have to change the TTS engine to something else then Pico. Do this by menu settings > voice input and output > Text-to-speech settings > default engine > (choose something else then Pico TTS). If you don't have another TTS engine, then download another one first from the Android Market.
2. Change the language to your preferred Google Navigation voice language by Settings > Language & keyboard > Select language.
3. Start Google Maps and choose navigation. Choose some route and you will hear the voice in your preferred language.
4. Change the language back to English. Because you like to have the OS text in English
5. Connect your Android phone to your computer by USB and open the file manager on your computer with the Android folder. Or open an Android filemanager at your phone where you can copy files between folders.
6. Go to the folder Android > data > com.google.android.apps.maps > testdata > voice
7. You will see two folders. One with 'en...' and one with the countrycode of your preferred voice language. Open the folder with your countrycode.
8. Copy the file voice_instructions.zip
9. Open the folder with en...
10. Overwrite the file voice_instructions.zip with the other voice_instructions.zip from the folder with your countrycode.
All done. Now you will have OS text in English and Voice navigation instructions in your preferred language.
Optional. If you like to have English back instead of your own language for Voice navigation, then delete the en... folder. Google Navigation will download the original file again.
Please let me know if this works for you. it was tested on a samsung galaxy s2 with dutch language. but i think it is android independent