1) they use a single, shared embedding space for the two languages, forcing the model to learn "semantics" independently (or rather, interdependently) of language 2) using back-translation for training. I'm not sure that I got this right, but this seems to be round-trip translation? So the model can self-assess its performance by checking the spanish->english->spanish difference.
Sounds very promising and interesting! However, it seems they only tested on spanish and english. I wonder if the similarity of the languages at the lexical level made these results possible.