Java looks more leaner, except when really wants to master it, also needs to understand JVM APIs for low level coding (invokedynamic and friends), annotation processors, JVM agents, just like C and C++ how the many existing implementations behave, bytecode rewriting libraries,....
> How can a language and its core library stand still
I guess by having a more abstract foundation and relying less on adding hacks like colored function? Haskell seems to be like that.
Another thing is language extensions that you have to turn on to use. This makes deprecating stuff easier in the future, so languages can trim itself instead of keep growing.