If you ignore human psychology, ... well, you're probably not making a great app.
Why have vanity badges, or other kinds of gamification, to begin with?
Because it gives users an added incentive, through an added reward, a goal, which helps them get through something that could otherwise be too tedious or un-fun for them to endure.
And if everybody agrees that the added goal aligns with the user's actual goal, that's a great strategy.
But you get what you reward, because people like to get rewards. The dopamine rush of knowing you'll reach the goal.
If the rewards stop helping users towards their real goals, and you've trained them to work towards your goals already, you'll get people doing daily tasks to get daily rewards that they shouldn't really care about. But they do. Because you made them care.
And you might have replaced their intrinsic motivation, wanting to learn a language, with an extrinsic goal, wanting to get a reward. And if they then stop caring about the extrinsic goal, they just might stop entirely.
Messing with people's motivations is dangerous, and "driving engagement" is the absolutely most useless reason to do so.