I would say so. We have many users currently using Errai for production apps.
Good to hear. I think a nice technical base, good community and active development are all indicators for a good project. But do you have any experiences regarding the two points in my question? Are your production apps localized?
Update: I found this: https://docs.google.com/document/d/1luJzaZzSwsLyt4dY_jyot1ERpJ8fC1lQD1VXG7Yof3g/edit?pli=1#heading=h.2p9pl1323cw which states internationalization support for Errai 3 (Oct 2013)
I'm pretty new to errai, but the two issues you raise have not been issues for us. It has been pretty easy to internationalize our app. And the cross-browser support seems to be just as good as GWT as a whole--I haven't found any differences. We've found that some things can't be done in IE8, but that is the limitation of IE, not GWT and certainly not errai.