Alten is a technology and engineering consulting company, which means, in the context of the French market, that when a company, need a particular profile (like java dev) to staff a position for a set amount of time (like for the expected duration of a project), they'll contract Alten for a developer, java, for x months.
Does it track market rates? I've worked for a company before that took fresh grads, and relied strongly on them never entering the market for themselves and realising what market rate was.
They're know for paying shit, and there are arrangements between agencies to not compete with each other.
Actual salaries at these places have been on a slow downward trend for many years.
The problem is knowing the market rate.
Do you have a period of time you are contracted to work? If you can make friends in the Paris dev community, you might be able to find out how good/bad your pay is.
And I'm too much of a lazy ass to look for another job. Also I have few reason right now to seek a higher salary.
Looks one you're one of them if that's what you've got? ;)