It definitely has solved the two language problem for me, I haven’t had to write any C code in 3 years now (after Tony Kelman, Steven G. Johnson and a few others pushed me from trying to solve some problems by writing a C library and a Julia wrapper, to doing it in pure Julia 
)
I originally resisted, until after extensive benchmarking, I was able to show to myself that I could get code as fast as C (and this was for low-level programming), with less work, by writing it all in Julia.
I tell people, when they say things like “One Language To Rule Them All”, that they misunderstand the original quote, and that Julia isn’t the language that will replace all the others (I don’t think that would be possible for any language), however, for many things, it could be the language that will bind them all together (hopefully not in an evil way like the One Ring!) via PyCall, RCall, Cxx, JavaCall, and the built-in ccall, and therefore rule them 