You have to look of engineer in mechanical engineering.
I have no idea what you mean here. I suspect that you could combine this with your next sentence, and say that "I am writing to apply for the position of mechanical engineer advertised in the July 25th edition of the Quotidien."
I'm looking for some time an opportunity to speak on a larger scale,
Um. You want to be an engineer so you can speak more?
Since the presentation to the world of advertising,
And now you are interested in advertising?
You need to focus more on what you have to offer the company as an engineer, and what you hope to accomplish in the position beyond generally gaining experience. Add specific details that will make you stand out from the crowd.