I am developing a website (with a mobile app for later), that helps people learn languages, but as I just started, I don't know yet if there is high demand (and growth rate) in language learning market. I'd also like to know what is the best plan to monetize the platform?
Before you get to far in to this, I would think about spending a little money on market research, maybe using a service like AYTM.com to figure out what kind of demand is out there for your specific service and what people would pay for it. Or maybe use SurveyMonkey and run some ads on Facebook to collect data, that is even cheaper.
I would also wire frame this and ask people (specifically, people you do not know) if they would use it, before you spend hours an hours coding something that may not be what people are looking for. Don't forget, your time is money as well. You should be as lean as possible during this phase of your idea.
Disclaimer: I am only a user of language-learning products and have not developed such a product myself nor have I ever contributed to language-learning products.
I think it depends on your approach. Will you provide something novel that makes your product stand out compared to your competitors?
There are many products already on the market that make learning new languages "easy", but they also differ greatly in features as well as in their respective approaches towards delivering learning material. I have personally discovered Babbel recently, which provides a fresh mix of different learning paradigms -- as web-site and app! Could you provide your own take on learning languages that excels in a particular way?
Also, I believe that you would be mainly charging for the content that you are offering, rather than charge to gain access to your web-site or app itself. I would suggest focusing on the mobile app first, because people probably want to learn languages when they are "on the go" (using public transport, during break times, etc.), but also because you can then make use of the already established in-app purchase systems that are offered for Android/iOS.
Paul graham of YC did suggest language app as potential startup in one of his essays, but everyone else is write, depends on whats special or unique.