XalanMsgLib adds quite a bit of complexity to the build and to the implementation. It's also completely unused: there is only a single supported language: en_US. There hasn't been another language added in the entire history of the project. The demand for and likelihood of getting any translations is near zero.
As a result, this issue is a proposal for dropping the XalanMsgLib library. If it's unused, why pay the costs for having it?
Unless there are any objections, I would suggest we drop it entirely for 1.13.