according to R Krohn (and others) in his book about naming a child, yiddish is secular.
Exactly what defines a language as secular or relgious? Just because today only frum Jews speak Yiddish (except for some old people) that was not the case in the past. Yiddish secualr culture has a strong history with the biggest classic being Tevye the Milkman (Fiddler on the Roof)