District 9 (2009) - 3 questions

Question: I remember seeing a trailer for the movie, in which a human man is questioning an alien. The alien tells him that they just want to go home. I just watched the movie for the first time and that scene wasn't in there. Was it deleted for some reason?

Answer: This isn't actually remotely uncommon. Trailers are almost invariably released before the final edit for a movie is locked in place, so it's not at all unusual for shots in the trailer to be from scenes that ultimately don't make it into the finished product, or to be from alternate takes of scenes that do appear. As to why this particular scene was rejected, who knows, most likely was just for pacing, or they might simply have not liked some aspect of the acting or whatever.

Question: Something I couldn't figure out. The black fluid (in the glass cylinder) made Wikus start turning into an alien. If the aliens were mainly interested in going back to their planet, why had they been developing a substance that could turn humans into aliens? How would this help them go home?

Answer: That's more of a side effect of the fluid. See, only the Prawns could use their technology; it was "genetically activated", so to speak. So the fluid which powers their tech is like Prawn-gene concentrate. When Wikus was sprayed with the stuff it affected his genetic makeup.

Question: Was there ever an explanation given for how the humans and the prawns learned each others languages? Both speak only their native dialects yet they're able to communicate with each other perfectly fine.

Answer: There's no specific explanation given, but it's not difficult to work out. A basic vocabulary could be worked out relatively easily simply by producing a series of items and learning the respective words for them. Over time, this vocabulary would build up until rudimentary communication was possible, which could then be refined over the decades since the prawns first arrived.