There is no city of license coverage requirements for an FM translator. A translator licensee can pick whatever city of license for a translator they chose. As far as "fill-in" requirements, the translators 60 dBu contour must EITHER be contained inside of the AM's daytime 2 mv/m contour OR contained inside a 40 KM radius (25 mile) of the AM stations transmitter site. This rule was relaxed some (from BOTH to EITHER) after some AM's complained about having tight daytime directional patterns being tough to design an translator's 60 contour to stay inside of an AM's 2mv contour, especially if the translator was collocated at the AM's tower site. The only thing the FCC later tightened up with many AM's now operating with temporary small coverage STA operations because of a lost tower site, that any associated translator 60 dbu must conform with the AM's STA 2 mv daytime operation or 40 km radius of its operation. While the 40 km radius is fairly easy to conform to with an STA, if its operating at a fair distance away from the licensed operation of the AM station. It can cause some translator to modify or shut off.As far as I'm aware, the only requirement is that a co-owned translator's service contour has to remain entirely within the primary station's 60 dBu contour (for FM) or daytime 2.0 mV/m contour (for AM).
If the translator and primary station are not co-owned, then this rule does not apply if I remember correctly. Feel free to correct me if I'm wrong about any of this.