That swc has the localized strings for rpc.
It should be there in C:\Program Files\Adobe\Adobe Flash Builder 4\sdks\4.0.0\frameworks\locale\en_US (or whatever locale you are using)
One thing to check may be if compiler arguments (specifically -locale) has been added/changed.
Thanks for your input, but apparently my failing hard drive corrupted the rpc_rb.swc file (among others). I downloaded a copy of the Flex SDK and switched to it, and that got me around the problem.