There's this paid app called Zenolingo. It's a simple one time fee and of course there's the Zen+ subscription you'll also want because you accrue ZenPoints much faster. They recently introduced ads but you also get ZenPoints from watching them, which imo is pretty fair.

You don't have to pay anything on top of that, but if you're running out on DailyZen then by buying ZenCredits you can top up your ZenAccount