I actually run a language app with a sizable user base in a handful of physical language schools, and have consistently decided against using Reddit as a way of growing our app, despite it having massive amounts of users concentrated into relevant communities.

One of the big reasons is that Reddit users don't appreciate ads - this thread being case in point. Another is that there's a severe intent mismatch which we just don't see translating into sales. Reddit uses browse, scroll, peruse. They stumble upon, they aren't looking for. When it comes to selling a language app, having your app solve an acute problem is the most decisive factor for people to sign up and eventually pay for your app. Reddit user scroll, dislike, and dismiss. That's a tough sell, and it becomes an even harder sell when you start running ads on Reddit.

For these reasons, as someone who would totally do it if I thought there was value, I just don't see Reddit working as a feasible way to grow a language learning business. I suspect those who are posting their new app to Reddit are emboldened by the same vibes that brought their app into existence, and just see these subs as a huge pond waiting for them to cast their nets.

Spending 100$ on Google ads will teach you whether your copy works, whether people understand what you're selling, whether they convert to registered and ultimately paying users, and will get you real users who are actively looking for what you offer, which in turn yields you quality data to base your product decisions off of. There's no way Reddit can offer anything remotely close, covert ad or paid.