A commendable result that you have launched and gained some traction not just with users but customers (paying users). Well done. However, if we take in to account the major levers associated with a subscription businesses:

- traffic

- conversion

- price

- churn

- lifetime value

The easiest one for you to change right now is price. This will have a noticeable effect on your revenue and your profit. At the moment, $700/year gross is not enough to retire on (I'm making some broad assumptions about your lifestyle as well as the costs of the business); it's also much less than the opportunity cost of offering your services to employers. I'm not dissuading you from running your own business, far from it. I'm suggesting that you should see greater value in your work - it's likely that others see it as well and would happily pay for it.

A few things to try:

- increase your prices until your gross revenue starts to drop

- a monthly subscription with a discount for paying for a year upfront e.g. 12months for the price of 10.

- price tiers for different 'dimensions' of the product e.g. x articles for $y, 5x articles for $3y, 100x articles for $10y

With regards to marketing, you may find better traction and traffic (which hopefully results in more sales) through education. Perhaps highlight how you and your existing customers can best use the app to actually learn languages i.e. what are the best methods, what techniques work well etc. I'm sure that you've got enough material for a blog post and even for a free mini-course in exchange for an email address.