I will try my best at this.
I did French at school, never used it, it's rusty, but I have a certificate that claims I am proficient. Should I disclose that my French is not much more than 'pardon' and 'madmoiseille'?
Are you going to be utilizing French in this position? Does your experiences lack so much that you need to fill out your resume? If so, possibly. I would explain that it is a 'Working Knowledge' of the fundamentals of French but it has been x years since utilizing French. The fact that you have to explain this though will look bad if confronted regarding it.
I have being working for a company every now and then for two year, do I put '2010-2011' or the concrete dates (which are many)?
I have been in this exact situation. I would list it as 1 block and break it down into multiple dates if possible or put something along the lines of "on an as needed basis".
I went for a couple of courses at a prestigious college A, but got a degree from college B. Can I put 'Attended college A and college B'?
Well, this can be viewed from 2 different ways in my eyes. Essentially, listing the college and a transfer of credits can be viewed as you couldn't handle the better college and went to an easier one or you were at least intelligent enough to get into the better college. I would try and avoid this situation personally but it really depends on the field you are going into and how important a degree and the particular school will be.
Generally speaking, avoid half truths because if the hiring manager suspects something fishy and confirms their thoughts, you are out of a job because of something that may not have mattered at all to begin with. Practice your interviewing skills and focus on the portions of your skill set that actually matters for the position.