In a remarkable display of dumbassedness, the application Monolingual will gladly strip all non-native code from your system. This is handy for knocking down the size of a few large applications but it doesn’t just hit your applications. It also targets the /System/ folder. Don’t every mess with the /System/ folder. Ever.

If you find that you can’t use Rosetta after running Monolingual, this is why. It strips out all the code in /System/ that you need to use PPC applications. The only solutions are to find someone with a working /System/ folder and copy theirs over (not recommended) or do an Archive & Install (recommended).

And don’t use Monolingual until they get this fixed.