“in heaven.” The Greek text literally reads “in the heavens.” While Jesus would have been speaking Hebrew (possibly Aramaic), and in Hebrew, “heavens” is always plural, the Greek text reflects this Hebraism that Jesus would have likely been using here. It is always best to represent the idioms of the culture accurately if possible in English. Sometimes that means replicating them literally and other times it means adapting them to the understanding of the reader.
“may your name be treated as holy.” This is a reference to the coming kingdom when the people will “keep My name holy” (Isa. 29:23) and Cp. Ezekiel 36:23.