Since it appears to be bugged in many savegames and profiles, I decided to upload my savegame that finally granted me the achievement. I've tried many builds, mods, reg/hex editors and nothing worked. After many tries the savegame from this playthrough did the trick. 

How to install:
Extract the file in you save game folder. Usually it's somewhere around Users\General\Documents\My Games\xcom - enemy unknown\xcomgame\savedata.
REMEMBER: Backup your previous save games.

I achieved this playing with the "Xcom Enemy Within Uber Mod" by FireHawkX. You can find it here on NexusMods. Installing it beforehand is probably a good idea, although I have no idea if its needed.
The savegame must be played on Enemy Unknown, not Enemy Within, and in English.

You should also probably prevent the game from "calling home" - you can find a guide on how to achieve that here. Disabling steam cloud save files is a good idea for the time being, too. (Right click on the game, click on properties, and untick the "Keep games saves in the Steam Cloud for XCOM: Enemy Unknown" box.)
After extracting the savegames to your folder, start the game. Then, tthe only thing you gotta do is research the "Interrogate Heavy Floater" project, which unlocks immediately due to South America's perk. That's it, the achievement should unlock immediately.

If you want to thank me for the savegame, you can head to my Steam Guide and drop an award :)

Thanks for your time!