Well, stupidly, as I was trying to mess around with the YouTube application to see how I could play around with it, I modified the following file on my iPhone without taking a backup:
and suddenly every bit of text in YouTube shows as "localized string not found"!!
I changed the text back, of course, but it obviously has a checksum which is partly based on last modified date, so it is still broken.
This file is not unique to any iPhone - it contains no user-specific or iphone-specific data, just a few english words (it's a resource file as far as I can work out). Could someone please send it/post a link/email it to me? Please!?