Now that might seem like an insane move, to film a series for a prestige cable network in a foreign language. But there's precedent, and that precedent is Netflix's super-smash Narcos. The streaming giant bet that their international audience would eat the show up and that Americans could deal with reading a TV show if it was of sufficient quality. Their bet seemingly paid off, which is why HBO is now following that model.
HBO, since the advent of HBO Now, has viewed itself less as a cable network and more of a standalone streaming service that wants to appeal to cord-cutters. That's led to a lot of shows getting greenlit that might normally have been passed over: HBO needs to beef up its streaming library so it can entice subscribers away from Netflix, Hulu, Amazon Prime, and all the rest. So HBO gets to have its prestige cake and eat it too, in terms of appeal to an international viewership that just didn't exist until recently. A savvy decision, and one we'll be seeing more of in the coming years.