Translation:Winter is settling in.
It would not be reflexive though. « J’installe quelque chose sour mon ordinateur. » This uses « installer »
This expression with winter uses « s’installer », the reflexive verb.
Sometimes a reflexive verb is similar to the original verb except that the action is being done to the subject itself. Sometimes a reflexive verb simply has a different meaning than the original verb.
"Settle in" or "set in"? My understanding has always been that "settle in" = "to become comfortable in a new environment" (after moving house, for example). "Set in" = "to begin, to be just starting". So to my mind, the accurate verb is "set" in the context of "Winter is setting in". Although "settling in" is a good metaphor, it has the the potential to mislead.