The only example I know of someone doing this is myself, in an edit I made to a question earlier today. I made an edit to the post to clarify some of it, and then later made a second edit translating the relevant parts of the error message into English. The second edit is still waiting for peer review, but thinking more about this I wanted to make sure I was not out of order.
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In general, if you see non-English content, edit away. I would be very wary in case of error messages, though: much can hinge on how exactly they are put; also, your free-form translation may not turn up anything on Google, while the original will.
Translate system messages only if you know what you are doing: ie., if you know for sure a free-form translation is not a problem, or you happen to know the exact correct English wording of the message.
Alternatively, translating the message and simply adding the translation is an option as well.
I say definitely do this, when the situation occurs.
The fact is, stack overflow in particular, is an English-driven site. Any time I see a post not in English, I just want to click away. Translating takes time away from me solving the problem at hand, and if someone will do that for me, I would be grateful.
Just make it clear what is the original fact-of-the-case error message, and what is just a human made translation.