Translation:The outskirts are very beautiful in this city.
Yes, it's singular, so contrary to other DL sentences (oamenii periferiilor sunt mai săraci) “periferia” here seems to mean the whole periphery of a city rather than just a quarter somewhere in the periphery.
Outskirts doesn't mean suburbs. Suburbs are often in the outskirts but a town with no suburbs would still have outskirts.
However, "peripheries" should definitely be accepted no? It means the same thing as outskirts but is a friendlier translation.