Where exactly are you seeing English? Some apps have their own language settings apart from the OS settings, if the guy only changed the OS settings, the other apps might still be the issue.
Are you using the You Tube app, or via a browser? If via a browser it could be because Adobe didn't give any new Flash support to Jellybean. (That doesn't mean you can't put Flash on it, you'll just have to manually install an earlier version of Flash.)
As far as getting a fresh copy of the OS - I haven't been able to find it anywhere. I searched extensively for it before I rooted mine, but never found it. Sony did some weird proprietary things to the Xperia OS, so it isn't as easy to rip or root as some other Android devices.
You might be able to use a custom launcher though, I'm currently using Apex launcher on mine and I'm pleased with it.