[aaggarwal]

But then wouldn't this require a bigger team with vast expertise or a lot of time compared to a more normal approach, and it will only result in marginal performance improvement, which might not be important in most of the regular projects?

[dllthomas]

"Best language for a task" isn't meant to refer to performance of code (except secondarily when the task is highly performance sensitive) but rather the language in which it is easiest to accomplish the task.

[dragonwriter]

It doesn't require any bigger team if the team are polyglots. It may require a more skilled team (it certainly requires a differently-skilled team.)