Let's assume you want to make $100k/year. Which is a pretty decent life in most places.
Right now the numbers are showing about $8.30/paying customer. To hit that target will require ~12,000 more subscribers. Not a crazy number.
But, at the conversion rate of .51% (126/24,717 uniques->paying) that means you'll need to get ~298 million uniques to your site (or 56 million signups). That's tough. Getting the entire population of the U.S. to come check out your site requires a huge marketing budget.
If you can convert all signups into paying customers (currently at 2.7% conversion) that goal becomes much closer. About 19% of all uniques are turning into signups (paid or otherwise). That's pretty good. If all of them were paying that's better than 1/3rd of the way there.
So really, for this to work for 1 full-time person to make $100k, conversions free->paid need to go up significantly, but probably pricing needs to go up as well and/or the tiered structure needs to be rethought.
Pushing the conversion rate from signup to paid closer to 100% means crippling the free plan, probably killing the chance of viral growth.
There's definitely room for optimising throughout the funnel though.
Also, I could look into raising the price, offering additional features, etc. An interesting source of revenue could be selling books via the site, although I have a feeling that dealing with publishers would be difficult.
That's pretty reasonable. Of course pushing the numbers up means fewer people will sign up at all (i.e. not every signup can be converted), so there'll be a trade-off of some kind.
At any rate, it might be worth setting a top-end of what kind of growth you might be able to expect and try and build your business model out that way. Right now the number of uniques you need to come seems a bit high to me. Might be worth modelling out what you could do with 1 million uniques, 10 million, etc. and try and model around that, then go out and try and get those eyeballs to your app. That kind of analysis can help you decide on marketing approaches. 10 million, for example, might require some investment in actual advertising, radio ads, late night tv etc. 1 million might be easier to do for "free" + your time.
Some things that might help get more eyeballs on your site
Once you're satisfied with where the site/product is, you might think about taking a few days and writing to sites like LifeHacker as well as language learning sites, any reddit about languages (e.g. /r/korean) to help kick things off (see below)
Other than time, these can be decent free ways to kick off some growth as well.
note: I've gotten some decent notice through the following in the past just by writing them with some information. Eventually most were able to carry it, and often it spilled over into twitter and drove traffic for a while while the news bounced around the globe. It didn't cost anything and was entirely free.
Also might get coverage in various newspapers tech sections, global news or travel sections.
$100k/yr would be very nice, although I'd actually be satisfied with considerably less, especially since this has value to me beyond the money - I think it's a cool project which will be useful as a calling card no matter what happens. If I could reach $2000/month sometime this year I'd consider it a success!
My wife took a couple years off to work on her startup. It never really went quite where she wanted, but did "ok" (a few thousand regular users). When we decided we needed her back in the workforce, having it on her resume was a source of intense discussion during all of her interviews and was a big part of her landing the job she has now.
People talk about knowing the entire SDLC as important for jobs, but running your own thing means that the software life-cycle is a really small piece of what you do and those additional skills are very valuable in the workplace.
edit btw, last night I thought about asking you if you'd consider adding a donation button on the free tier. People may be intermittent users, think it's useful, but not want to commit to the paid tier, so they give you a donation instead?
It could be a way to transition somebody up to the paid tier later if they're regular donators. Maybe keep track, and if somebody donates a certain number of times, or a certain amount of money, offer them a discount for a year or something.
I doubt there would be enough donations of less than $4.99 to make it worth adding the option.