I was watching French in Action a few years ago, and thought that this might be a good way to introduce an alien species to humanity and a particular culture. It's a foreign language telecourse that's produced entirely in the target language of French, so in order to be effective, it needs to demonstrate the usage of the language, as well as every concept a human being needs to talk about. The only problem is that this approach would inevitably focus on one culture, no matter which language you choose.

I do agree that it's completely pointless to send out a single one-time message. It seriously needs to repeat many times, for a long time. Who would pay for that?