I'm making reference – in a piece in English - to the ‘Bibliothèque nationale de France'.
Now, I'm pretty sure that that is the correct way to punctuate the name *in French* - which, in such compound names, generally rejects the English standard of capitalising every significant word.

But – even assuming that's right – it does not follow that the same rule should apply in English.
It seems to me that any translation of the name into English - ‘National Library of France' – would be a ‘surtraduction': if the context did not make clear what one was talking about, better to use the name in French and provide an explanation in English.

Leaving the choice of the original in its original punctuation – which looks pretty odd in English – or a hybrid: words French, punctuation English.
It sounds like the sort of question that a newspaper style guide would cover. I've looked at the ‘Guardian' version (1) – but no joy. (In fact, it has some pretty free-and-easy suggestions on capitalisation of English words!)
(1) http://www.guardian.co.uk/styleguide/0,5817,184913,00.html