For $10 a month, subscribers to Duolingo Plus will get access to an ad-free version of the lessons they already use on the language platform, plus the ability to download those lessons for offline use.  The company says it's ideal for users who want to be able to practice during their commute or on plane flights -- but stresses that it's purely optional.  Free lessons won't be going away.  Technically, a subscription model contradicts the company's original promise for a service that both teaches language while simultaneously translating the web, but the firm says it's an important step.  According to a statement given to TechCrunch, if everything goes to plan, the company will finally be able to break even by the end of the year.