[JonChesterfield]

That's the benefit of mixed language projects.

You're skipping over the cost, which is in reliably and efficiently passing information back and forth between languages. It's generally some form of serialisation. Occasionally both languages get to run in the same address space which moves the complexity around a little.

There's also the human scale drawbacks of having predominantly C++ devs write JavaScript or vice versa when people should touch the far side of the language barrier.