Quoted from an email sent to me:

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I've had three people from CUSEC now tell me that they have or could learn Ruby very quickly. One guy even said it would be "simple".

Now, I understand what it is to be a polyglot! Knowing many languages helps you solve problems. Yet it is a significant faux pas to imply that someone else's tool of professional choice is simple/easy. You could be Turing or Einstein, but that's not the point: it unintentionally patronizes the recipient. It says "that skill you've been perfecting for 5-10 years? HALF AN HOUR, BIATCH!"

Sure, you can sit down and see most semantic or syntactic differences of a language very quickly. If you have the background, you might even dive in to very advanced subjects as a means of comparative evaluation.

However, I don't think that you can truly know a language before you've used it to solve problems over time. That's because a language is not just syntax anymore; it's the culture, community, tools, history, people, libraries, resources and last but not least patterns and best practices.