Need some advice on the specific groups/sites/community where I can promote my language web app?

Hi everyone,

I've built recently a web app called kotoha (www.kotoha.io) which is a Multilingual Collaborative Urban dictionary, where people can submit useful phrases, slang expressions that are kind of hard to find in textbooks, and got them translated (ie. find a native equivalent expression in another language) into other languages.

I didn't spent a lot of time trying to promote it online, except for a few posts on Reddit in subreddits like /learninglanguage or /learnjapanese. These posts ended up at the bottom of Reddit within a few days, or get deleted because of self-promotion (even though I asked permission before sometime it get deleted).

My posts got some upvotes and managed to get around 50 users since its launch a few months ago (but really, I didn't invest much in trying to grow it at all yet).

But so far, I'm almost kind of the only user on that lol (yep eating my own dogfood)

So my questions is: how do you find those specific niche of users ( ie. language learner)?
I am aware of some Facebook Groups, but again it's most of the time against their policy to self promote.
I know that a lot of people are actively more or less learning some foreign languages, but it's kind of hard to find them and get my message through without being spammy and got banned somehow.

Any feedback, advice are really appreciated!
Be it about the tech, design, growth etc...

Thanks you!