Nowadays English is the most common language in the world. Knowledge of this language will open you a lot of opportunities.
In my opinion, the main opportunity is to communicate with other people. Like I said before new connections open up new opportunities for you, increase your horizons and thanks to this you will learn about the life of other peoples. And all of these opportunities will open for you if the world will be one language. But also it is a flip side of the coin.
Thanks own language countries and nations have their own specifics, that highlight them from others. For example France it is very popular thanks to their own language, and specific of their language is no pronunciation of the letter "l". In addition, I want to say that a lot of jobs will disappear if the world will be one language and it makes some people poor and unhappy.
In conclusion, I want to say that there a lot of advantages and disadvantages, but it will be better if it is will be a lot of languages.