in DNN7 i used to have multiple css for each language by adding the condition

if (System.Threading.Thread.CurrentThread.CurrentCulture.Name == "en-US") {

ClientResourceManager.!(this, ctlSkin.SkinPath + "skin.css", FileOrder.Css.SkinCss);

//add CSS links

//ClientResourceManager.RegisterStyleSheet(this, ctlSkin.SkinPath + "skin.css", FileOrder.Css.SkinCss);

ClientResourceManager.RegisterStyleSheet(this, ctlSkin.SkinSrc.Replace(".ascx", ".css"), FileOrder.Css.SpecificSkinCss);

}

else {

ClientResourceManager.RegisterStyleSheet(this, ctlSkin.SkinPath + "skinar.css", FileOrder.Css.SkinCss);}

but nothing is changed in DNN8 i think its not compiling my cs