I have never learnt a language in SL, but would love to try!
I have read some research on it and it seems to be a medium that very much favours language learning – authentic practice, strong context (endless possibilities for simulations and role plays), strong levels of identity and even masks to hide behind for more shy students
I see a moodle environment as a great place to start learning a language – controlled environment with immediate feedback. In fact we also use it for intermediate ESL and EFL students for skills and vocabulary development.
I envisage SL as more of a place to practice language in a less controlled environment that is semi-safe, but challenging enough.
That said, maybe it is also a good place to learn the basics with drills, repetition and so on? Great potential for speaking and listening - but not sure about writing and reading skills.