When I started my car this morning the RNS 510 unit started up in German and the radio wasn't playing. When I try to change it back to English it says something along the lines of 'the selected language is loading (in German)' but nothing happens after that so I'm guessing its crashed. I've tried the 3 button reset and disconnecting the battery but still having the same issue.
Called Arnold Clark Volkswagen and they said it may be covered by warranty but they couldn't tell until they checked it. If it isn't covered I would need to pay the £75 diagnostic charge
The RNS in my old GT used to revert back to german every now and then when I was watching a DVD. It used to reset itself eventually - how long have you left it it?
That said, watching south park in german was pretty hilarious (sorry not helpful I know).
I managed to fix this by updating the firmware (to 5274). The unit was still in German but after pressing Setup>System>Language then changing the setting to Automatic now changes the language (it didn't change before).