I think it depends on how you look at the language. For instance, I wouldn't say I've learned
Smalltalk, in the sense that I would be comfortable writing a proper application in the language, but I've spent enough time with it to get a much deeper understanding of the possibilities of a dynamic object oriented language (like, say, Perl). It also helped me come to understand what it is I don't like about Java and C++ (Static typing is just wrong! OO without blocks/closures isn't really OO! Surely if you're designing a language with OO as its central feature, it would be a good idea to come up with a nice, concise syntax for making use of that feature. Ahem).
My take on Andy & Dave's advice is to learn enough of a programming language that you can read good code written in it, then do that. Find books that are well respected by people in the language's community (for example, SICP for scheme, On Lisp for common lisp, Smalltalk Best Practice Patterns for Smalltalk...) and read them. You're not trying to become an ace programmer in your new language; you're trying to get a feel for the mindset that goes with the language.