As I point out in What happens during the process of translation?—
[…] a good translation is [one where] you write the text as the author would have if they were a native speaker of the target language. The translated text should emulate the original in terms of the level of literacy, style, and (if someone is being quoted) the socio-economic/educational background of the speaker.
Since not all texts are written well, however, sometimes one must also engage in a bit of farteischt und farbessert (from the Yiddish, “translate and improve”), to convey the writer’s meaning as cogently as possible.
If a translation is meant for publication (or some other form of transmission), anything that clarifies the intended message is welcome.
In my case, I go beyond merely correcting my clients’ work, and maintain an entire blog (Al Fadiḥ—Arabic/Hebrew slang meaning ‘Don’t Screw Up’) on the pitfalls of writing (especially in English), and how to avoid them. This both helps my clients to avoid such problems in future, and serves as a handy online reference when I point out issues in the translation at the end of a job.
The notable occasion when one must not correct the defects of the source is when the writer is quoting someone else—be it another text, or a talking person during an interview. In the former case, any change from the original is strictly verboten; in the latter case, the errors in the original must be emulated in the target language, to convey the same impression of the speaker’s fluency and/or educational level, such as this excerpt from the account of a teenage girl in Israel who has run away from an abusive home:
I was just very lucky: I’ve had a lot of social workers in my life, but it was like, so I had this one good social worker, who was very good—like, really, really good, and she didn’t, like, let me be on the streets for a single moment, or in one place—so, it was like, “Until you—until I find you somewhere else…” It’s like, she didn’t let me leave [home]. That’s it.