I know that there are prisons where the interaction isn’t face to face, rather via phone where the only thing separating both inmate and visitor is a window (from that type of interaction, the conversation is definitely recorded but can languages alone hide the nature of the conversation?) That depends on the language spoken and it’s similarities to English (like Spanish) while languages that are very different (like Japanese or Korean) making it hard to translate.
I mean, most languages can be translated but the nuances from the mother tongue won’t be 100% retained since changes can be made plus different languages have their own cultures attached to them. As in what is considered profanity and offensive in the mother tongue translates as a normal word in English (unless you clarify the context), as in what is 100% accurate won’t be to the other party since languages are different the same way cultures are.
I think this depends more on who you are, where you are, and what you’re in for.
If you are a dumbass who got put away for stabbing someone who looked at you weird at a bus stop, then you can probably make your plans in plain english. No one cares. You don’t have plans. This is obvious, because you knifed a guy for looking at you weird. You got put away in the podunk prison for dumbasses. Sure, they can listen to your calls, and might even do it if they are bored. But mostly the prison is trying to make sure everyone is fed well enough that they don’t start a riot.
If you are the head bookkeeper for an organized crime syndicate, on the other hand, you can be sure that the FBI will be recording you every time you take a shit, and will have a team of translators ready to translate anything you say, along with slang, jargon, cultural cues, and unusual emphasis.