Here is an overview of the average performance at open test. But we shall not extrapolate too fast… Presumably the most difficult will be from 98% to 99%, and even worse from 99% to 100% (if attainable…). It could even be extremely difficult to pass from 99.00% to 99.10%… And it may even take two years to pass from 97% to 98%. Who knows?
An open question is: how hard will it be to pass from 98% to near 100%? It seems such an ability would require multi-language translation capability, and it could even require full-featured AI (not limited to machine translation). To put it more clearly: will near human quality translation require full-feature AI?