I seems to me like the current implementation of the preferred translator system is not very intuitive and in many cases not productive.
For example customers might think that the system doesn't work at all because they don't realize they have to add another preferred translator, or they might shy away from the "work" of having to assign a second one when they have just found one translator they are happy with. (Not trying to imply that there are actually jobs that can be only done by one special translator, but in some cases the customer might want to have just one translator e.g. if consistency is especially important.)
Also it seems like some customers will promptly remove you from their list of preferred translators when you aren't online for a day or so and their translations take longer than expected. (This is just a guess, though, I'm not 100% sure how the system works.)
So my suggestion would be that the preferred translator system works from the first added translator, but the customer can set a deadline after which the translation is open for all. (Which should be visible to the preferred translator(s) so they can individually judge if they expect to be able to start the job within this deadline or release it immediately.)
It might also make sense to send out a special email to the preferred translator if a preferred job is coming in, so he can filter these emails accordingly.