The official term for those used by law enforcement to gain the confidence of criminals and report back to the police/authorities is 'jailhouse informant'. Of course, there are a number of less official terms such as snitch/rat/stoolie/squealer/snout (UK).
The term 'canary' is sometimes used as birds 'sing' and this is the verb often used in underworld slang in reference to snitching to the police.

This is a similar technique, that was common in Canada. it may give you some ideas towards what you are looking for - see the WP article

Options. Plenty of choices. See refs.

Hi, Frank. Customize per context. I've looked at pages about Illinois v. Perkins, which is the major US case on the subject, and I haven't found a specialized term. I imagine that there is an informal English term somewhere or other, but the Hebrew term appears to be formal, and I assume that if there were an accepted formal English term, it would be in evidence.

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Note added at 4 hrs (2020-07-15 18:09:18 GMT)
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I'd be wary of making up a term for a concept that isn't novel in the target language. I wouldn't use "informant," because that normally refers to someone who doesn't belong to the given organization. Better to write "undercover police officer" or such, and provide the prison cell context if and when necessary.

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Note added at 18 hrs (2020-07-16 08:31:15 GMT)
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"Plant" would work, but only in an informal context, which I think this is not. (Keep in mind that the word is מדובב, i.e., one who causes to talk, rather than דובב, i.e., one who talks.)