"It is the custom in New York to sing sweetly."
Translation:Mos Novi Eboraci est suaviter cantare.
With cities/towns and small island, to indicate a place, we don't use "in...", but we use the locative instead.
So "Novi Eboraci" is a locative meaning "in New York".
But "Mos Novi Eboraci" as the custom of New York, also makes sense in this sentence, so it could also be the genitive. But it's not always the case. As a rule, when the locative is possible, try to translate it with a locative, before trying with the genitive (when both are the same, of course).
It can be both here, locative or genitive.
Like for instance "Novi Eboraci Civitatis sigilum" means "The sigil of New York city", or Romae mos.
Mos Novi Eboraci = The custom in New York or the custom of New York, both makes sense.