Way easier. It is hardly noteworthy for a programmer to know a half dozen programming languages well, but a person who knows a half dozen human languages well is.
I think the main determining factor here is vocabulary. Modern programming langs have fairly small #s of reserved words, and when you add the standard library on top of that, you essentially have all the 'vocab' you need to use the language well. This is because (a) a large part of programming is the act of creating new vocab via writing new APIs/libraries/functions, and (b) the time pressure is significantly lessened; there's no issue with consulting docs for the interfaces you forget.
Meanwhile, human languages are essentially unusable without a fairly extensive vocabulary, and importantly a vocabulary that has to be memorized and available at a moment's notice.
One needs a lot more fluency than the other - there's a difference between what could be considered 'knowing' a programming language (where it's possible to slowly cobble together, or decode, a sentence over an afternoon with documentation to hand) and conversing 'live' in a natural language.
