I'm not sure there was ever a time where it was good to be a one language shop. Although people continue to pursue it, I haven't seem much progress in creating a language that is both dynamic / abstract and fast.

In my opinion, you usually need two; a high level language that supports powerful abstractions and terse representation of the problem and a low level language that is good at expressing things in terms of hardware operations.

I choose Io + C, but any two that fit this description should be able to cover most cases.