What should they know of PHP who only PHP know?
Winds of the World, give answer! They are whimpering to and fro —
— Rudyard Kipling – The English Flag
And what should they know of England who only England know? —
The poor little street-bred people that vapour and fume and brag,
They are lifting their heads in the stillness to yelp at the English Flag!
I like that stanza a lot, and (taken out of context) it rather suits an opinion I hold on the merits of learning of several programming languages. Most programmers have a core language, the one they are most able and comfortable with; I think we should expand the list of peripherals.