I think I have command of a language when I can write something from scratch without spending much (if any) time in the docs looking up things.
How many languages have you learned before Python? The first is always hard, and the second is always the worst, because it's the first time you have to think above the level of syntax, but after that it's much easier. By the time you get around to learning number nine, all you have to conceptualize are the diffs with what you already know, which is a much smaller learning curve. So, to summarize, it gets easier with each one you add to your repertoire.
As for problems, I just start with the current one. I usually learn new languages because it seems well suited to a problem I have for some reason. Right now I'm really interested in parallelism, so I'm learning Erlang. I'm working on writing some kind of REST ROA stuff with it (and Yaws), for fun.