What if you were offered free tacos and tortillas as long as you spoke Spanish when ordering in a restaurant? Your ability to speak the language of Cervantes would improve in no time, wouldn’t it? That’s the aim of a soon-to-be-opened restaurant in the United States. It’s a PR stunt — we’ll give you that. But you have to admit that the idea is particularly well thought out, not only to get the word out about the brand, but also to prove that the restaurant of the future will be dedicated to multitasking. The Duolingo smartphone and tablet app, which boasts 42 million mobile users a month, ha…