My D-Day

This morning I have started studying English because I have to know this language very well [meglio dire "I have to be fluent in this language" se non cambia l'intenzione] within twenty-one days.
I'm having I have a job interview on September 4th and I should have a good knowledge of spoken and written English. The problem there[superfluo] isn't the interview because I think it will be in Italian, language but, it goes without saying that I should know English, because it's a vacancy for a secretary in an international law firm.
I like challenges so I have decided to speak a fluent English,Spanish and French languages before onewithin a month.
I have decided that foreign languages come quickly to me quickly.
I think that whoever knows foreign languages can be the master of the world. [Allora, se diventi Master del mondo, ho alcune richieste.]

My D-Day

This morning, I have started studying English because I have to know this language very well within twenty-one days.
I'm have a job interview on September 4th and I should have a good knowledge of spoken and written English. The problem there isn't the interview because I think it will be in Italian language but, it goes without saying, that I should know English: it's a vacancy for a secretary in an international law firm.
I like challenges, so I have decided to speak a fluent English,_Spanish and French languages before within one month. {more natural: "within a month"}
I have decided that foreign languages come to me quickly. {more natural: "come easily to me"}
I think that whoever knows foreign languages can be the master of the world.

My D-Day

This morning I have started studying English because I have need to know this language it very well within in twenty-one days.
I'm having a job interview on September 4th and I should need to have a good knowledge of spoken and written English by that date. The problem there isn't the interview because I think it will be in Italian language but, it goes without saying, that I need to know English, since it's I have applied for a vacancy for job as a secretary in an international law firm.
I like challenges so I have decided to try to learn to speak a fluent English, Spanish and French before in just one month.
I have decided that foreign languages come to me quickly.
I think that people who know foreign languages can be the a masters of the world.

Viviana, this is so impressive and I am inspired by your confidence! The changes I made bring your text closer to American-style English (although no American would be so confident, not would they be required to learn any languages!). If you are interested in speaking practice, I would be happy to help you, as I am trying to learn the most beautiful language in the world (Italian) and I need your help for speaking practice.