Japanese language learners congregate in a bunch of online discussion boards and subreddits. Some of my competitors have forums that are open to anyone posting about their own self-made tools, in addition to users discussing learning resources unrelated to the host's products. So I simply posted about my service on several of these and quickly gained thousands of early users.
I had more luck than others who try the same because my product solves pain points and offers features that competition don't or don't as nicely, ie quality and value. I also attract users with friendly pricing: a standard pricing tier and an unverified student / low-income pricing tier for the same service level.