That's a pretty cool idea, at a base, create something useful and force yourself to look through the standard library. I am learning GO right now and I am blown away with what comes out of the box.
I think this is a great way to learn the ins and outs of a language once you have gotten past the basic syntax differences. Going from say Ruby to GO / Clojure there is a big mental paradigm shift on how to solve a problem, so taking an existing code base in which you have a good grasp on the domain knowledge absolutely helps. I find looking for [insert language] Koans or trying some Project Euler challenges to be very effective as well.
My opinion on the idioms, best design practices, etc... when learning a new language may differ. I almost always feel like some people just understand everything and I am always running to catch up. It turns out no one was born to think in base2 and these individuals just spend a lot of time reading the docs, hacking around on small apps and really taking the time to understand not the how but the why. So I often have to convince myself, you will get there too it just takes time and practice. Rarely does anyone learn a new language in a weekend and put out top notch, idiomatic code, and if you do come work for me so I can sit back on the beach all day. No one picks up a guitar and shreds out Eruption from Eddie on day one, they play some basic Nirvana chords.