We have the Garmin Nuvi 370, which also has the European maps preloaded. I believe it is the same as your 670 except that the screen is smaller and the price is also. I also bought the language guide, but from what I can see I don't think it's going to be of much help. We only speak English (despite living in Miami, FL) and hope that we'll encounter bilingual people in our travels.
Per your warning, we won't attempt the trains in Marseilles, but may consider renting a car. Or we may stick with the over-priced tour we've signed on to with Princess.
Thank you again for the very helpful information.