Also Romansh: Source: "Vornamen in der Schweiz. Prénoms en Suisse. I nomi in Svizzera. Prenoms in Svizra" (1993) published by the Association of Swiss registrars Https://www.portraitarchiv.ch/portrait?page=219 Http://www.annalas.ch/persunas/display/q:Silvester
Looking at records of births and christenings in England from 16th to the 20th century, it seems that it was sometimes given to girls.EXAMPLES (accessible via Ancestry & FamilySearch): Silvester Rogers (christened in 1583) Silvester Ann Scott (born in 1862)So in that sense, both Silvester and Sylvester were unisex names back in the olden days, though given more, slightly, to boys.
Source: "Vornamen in der Schweiz. Prénoms en Suisse. I nomi in Svizzera. Prenoms in Svizra" (1993) published by the Association of Swiss registrars
Https://www.portraitarchiv.ch/portrait?page=219
Http://www.annalas.ch/persunas/display/q:Silvester