translate unified_inventory button mouseover text

unified_inventory_page.lua

@@ -12,7 +12,7 @@ unified_inventory.register_page("skins", {
 unified_inventory.register_button("skins", {
 	type = "image",
 	image = "skins_button.png",
-	tooltip = "Skins",
+	tooltip = S("Skins"),
 })
 
 local function get_formspec(player)