Phrase for "Night Haystacker"

"The lazy donkey kills himself with one load."

In stead, he should go back and forth, carrying smaller loads, but still effectively get the task done--only, he does not want to take so much time, so he carries it all, and risks breaking his back.

A quick Google search doesn't show up any relevant results, when I find something I'll update. However, my aunt Eileen, who was English, and my grandmother, who is American, both used it.

"... bit off more than he could chew"

This has some of the implication you're asking for, in that someone attempted to do too big of a job too quickly and found themselves unable to complete it. It doesn't have the "heavy load" part of the original phrase, but it's metaphorical. It can also apply to a situation where you rashly jump into a scenario you're unprepared for, e.g. the soldier that charges into a firefight against what first seems like a small group of enemies and then realizes he's stumbled across a much larger force than he was expecting and is hopelessly outgunned.

Glutton for punishment seems to fit some aspects of what you're looking for. Someone who is a "glutton for punishment" eagerly takes on tasks that others (or even himself) might find unpleasant. It might be because the person is incapable of saying no, or because they are "paying their dues" by doing otherwise undesirable tasks, or it could even be the case that the person actually enjoys these tasks (although in that case, only other people, and not the person themself, would use the term).

It might fit your desired usage of a translator who consistently chooses bad clients with short deadlines - they might have the power to choose their jobs more carefully, but since they are a "glutton for punishment", they keep coming back to the same patterns that result in a poor working relationship. The phrase mainly connotes the habitual nature of self-inflicted, unpleasant situations, but it doesn't address the desire to quickly exit the situation.

I am not sure this is good enough for you but you might want to try working with the idiom have too much on one's plate.

Have too much on one's plate: take too much responsibility/work etc. that is difficult to "digest" — TFD

I think you can make a few tweaks to the idiom's structure, eg. have/get/take...; maybe on sb's plate as well (?).