Is Duolingo a reliable source to become fluent in another language?

Not to become fluent. But it is good for practising and learning some new words and phrases. But you also need library books, reading material, tv shows, movies, radio, a good teacher. Best thing to have is a native speaker to practise with

That looks very odd. It says "Learn languages completely free" but then it says "All language courses 50% off!" So I don't trust them.

Anyway you won't get fluent with an app. You need to spent a lot of time speaking and listening to people.

You will need another source of learning, but that helps you learn more vocabulary words. Take my word dont use it for grammar, only for vocabulary :)

No, but you can learn a language in a fast pace on that app.