Ask your potential providers what systems they have in place to manage complex translation projects, assign various tasks to different people, and track the progress of each of these tasks. Do they assign a project manager to your project who assumes responsibility for delivery, provides milestone reports and has the entire scope and all minor details in view? Do they provide online status reports of your project?
A top language provider will go above and beyond the call of duty and foresee your future needs. For example, if he knows that you update your catalogue every quarter, he will create a project-specific process which will automatically flag the segments that need to be translated every time, instead of having to re-translate the entire document. This process not only saves time and money but builds in consistency across the board and eliminates the possibility of errors.