It's a rare name in the Slavonic languages, but юна (juna/yuna) appears to be the feminine singular of "young/youthul", Old Church Slavonic юнъ "Junu/Yunu" (spelling with a Y is according to English orthography). It's also the archaic Romanian feminine of june "youth/youthful" (from Latin) (pronounced yuna, but spelled in Romanian juna). The French dereivative of the Latin word is Jeune, which happens to be the French form of the Breton St. Juna/Yuna. There is no substantiated origin for the latter name, but there is a vague suggestion in claims the French form is
Yvette that it may be linked to
Iona, as if related to Breton iwin, Old Cornish hiuin "yews".