I am studying engineering at EPFL. I am going to improve my second language skill, which could help me to find a job after my graduation. For finding an engineer job (mechanical and materials engineering), would it be easier in German part than French part? Should I improve my German or French? Well both are impossible since i only have 2 years left.

what do you think?

Given that it apeears you are studying full time at the epfl since a couple of years, i'd hope you are fluent in french... Or not? If not, you have a pointer right there...

I believe the biggest employer of Mechanical Engineers here would be Alstom. They have a huge HQ in Baden, so German would be good, however they are actually a French company, so French would give you a boost when dealing with the French management. In general most engineering jobs in Switzerland are in the German speaking part.

Are you sure you are studying there? last time i checked there was:

1- Mechanical Engineering
2- Materials science and Engineering

I don't understand the part in bold.
Qu'as-tu choisi comme filière?

well my topic combined materials engineering and mechanical engineering.
it is not a particular major.

i stay in epfl one year now. learning french is not that easy job. well learning any language is not easy to me.

Well learning the two languages together will distract you, and yes learning the common language of your futur employer (if you know who is it) is great - but it is not the only thing that will help you to have the job.

Dont spend much time thinking and deciding - if you select the French, the advantage will be that you have all the day long to practise it with people around you.

If you practise German, then you still have the possibility to practise it but don't forget that the German Market then will be also an opportunity especially when it comes to Mechanical engineering.

Good luck, decide now and what ever the decision you will never loose as you will gain new experience in all cases, the only loss is if you stayed hesitant till the two years run away.

I am finding it hard too. I am a junior telecommunications engineer looking for an opportunity in Switzerland but after a hundred CV´s sent I had only two phone interviews

Both gone wrong. One asking for more experienced other asking for a good level of french. I have good english but the local language is mandatory too.

Although Alstom uses English as their language of business and while the language of the coffee room might be dominated by German speakers, the language of the meeting room is English (or some approximation thereto), and plenty of people who work there have German (or French) skill that are somewhat ... lacking.

yes, local language is preferable because of tight competition. In general, german language is preferable for technical area than french.