I think XLIFF format could be very useful addition for the library. It has the same benefits as PO files - it's standardized, widely supported, but beside of that it's also supporting ids in addition to the source string which is PO file doesn't.

I've started implementation of this one, but due to lack of time and interest (i'm not going to use it myself) i don't know when I would be able to finish this. So i want to publish this as is, and maybe someone picks it up where I've finished.