"I am in the restaurant."
Translation:Io sono nel ristorante.
Hm no, unfortunately the Italian prepositions have a wide range of usages, and the same type of prepositional phrase can be introduced by several of them depending on context. For instance the "complemento di moto a luogo" (i.e. the prepositional phrase related to movement toward a place) can be introduced by (quoting https://it.wikipedia.org/wiki/Complemento_di_moto_a_luogo) in, a, da, tra, su, verso, and per; among those only "verso" is strictly related to movement (or rather direction) and properly translates "toward" (I'm used to consider it an adverb rather than a preposition though).
When you use "sono" you aren't referring to any movement, so in Italian it would be "complemento di stato in luogo"; it can't be "I'm off to the restaurant", but it can colloquially be "I'll be at the restaurant".
To "I am in the restaurant" the answer "Sono nel ristorante" is less correct than "Io sono nel ristorante", given the fact that the sentence in English has a defined person. Although "Io sono nel ristorante" is taken has a wrong answer. This is not the first time this error as occored. Correction in future updates should be considered