No Stupid Questions

ALL recordings can be decoded later.

No exception possible.

The only thinkable "workaround" (not always applicable!) is when information becomes worthless or useless after a relatively short time, and the possible decoding would need more time.

They just get a translator, and you're screwed. No amount of complexity between any two real, actual languages will make this "good" encryption. That definitely includes Japanese, most of whose quirks you could get lectured on by any teenager with a passing interest in weeb culture. What you want is encryption, ideally something that can pass below the radar, languages don't do that. Even the hardest, clusterfuckiest of constructed languages don't do that.

It really depends who is recording you and their resources... Is it Tom down the street who you're hiding the weekend get away from? Yeah, switching languages have a high likely hood of working. Is it interpol who you're hiding a human trafficking operation from? Nope, no chance simply changing languages will work.

The only time I heard of this working is back in WW2 with the Navajo Code Talkers. The Germans couldn't (or had a very hard time) translate the conversations at all. Not a history buff so I could be wrong on the details.

Best to use an E2EE messenger.

In general this isn't going to work with any language that the people reviewing the recording can find a translator for, however a variant of the idea did serve the Allies in WWII. Native Americans were drafted into the military as "code talkers". They used their native languages to send and receive radio messages, and, as the Axis powers did not have access to anyone who spoke those languages, they could not understand intercepted messages.

Although no code talkers was ever captured, it is alledged that a member of each unit they were attached to was given a secret duty to shoot them if it looked like they would be, in order to protect the code.

Heavy dialects could be an option to make it at least more difficult for those who listen. My step father is from Southern Germany and speaks a moderate Swabian dialect making it difficult for people in mid Germany to understand everything he says even though the basically speak the same language.

He once had car issues in Hesse and wanted to ask the mechanic if it's okay to leave his car where it's currently parked. What he said was "Ka' i den do stong long?", the generic German equivalent would be "Kann ich den da stehen lassen?".
Assuming both people know the dialect and are able to talk it fast it may be pretty difficult for a third party to understand fast enough what's going on.

If the people spying on you are motivated enough, they'll find someone fluent in that language to fully translate, including the nuance.

Maybe if you used an extremely obscure language there there just aren't many people who speak it, they would be unable to find someone. But that just becomes an arms race with you trying to find a language obscure enough to overcome their motivation to spy on you and/or their available resources to find a translator. It's a lot of up-front effort for you, presuming you don't already know the language and have to learn it ... and it's far from a guaranteed success.

You may have better results with an invented language, one that only you and the people you're talking to know. And the less similar it is to any existing language, the more difficulty they'll have in trying to decipher it. Still, not a foolproof plan -- if they record enough conversations and pay some intelligent linguists to analyze it, they may begin to be able to decipher at least parts of what you're saying. And even more up-front effort now: you have to invent a whole language and everyone in the conversation has to learn it.

The more reasonable and practical way would be to learn what sort of speech is and isn't legally actionable and then tailor your conversation to avoid the legally actionable part. Think of the mob boss telling his underlings 'This guy bothers me. I wish he would go away.' His underlings understand that as instructions to go kill that guy. But it's vague and remains plausibly deniable in court. If that conversation was recorded, the recording would not be slam-dunk evidence against the mob boss.

Try Cockney rhyming slang

Any org with wiretap capabilities can affort translators. If you used an esoteric language, it might take longer, but likely won't change the outcome.

Also, if no one knows what you said, you can't prove what you didn't say either. So a less ethical organisation could simply claim that you gave a kill order, when it was actually a takeout order. You cant defend against it without also breaking the code.

I think the best outcome is for your defense attorney to sow doubt over the recorded audio.

There are a number of things a recording will not pick up. Somebody could make air quotes with their hand while giving the spoken word a neutral tone. You could easily get to a he said she said situation there. Did they mean it? Or did they know about the wire? Do we have witnesses who saw the air quotes? Reasonable doubt spreads from here. I saw an interesting video where a mobster was showing his technique of pouring wine in different ways to communicate to his in-group to watch their mouths if an outsider is there. A wire would miss stuff like that. It's the sign language of House Artreides, for the people who don't hate sand.

There is also something to say about translations. You'll inevitably arrive in situations where different interpretors will translate certain quotes differently depending on context. There is wiggle room. So the hypothetical Japanese wire recording could provide some fertile soil for reasonable doubt in an English speaking court as well. Less so because of the additional politeness levels but because the language drops subjects all the time. English grammar slavishly demandsa subject for most phrases and the Japanese omit them willy nilly. Who did what and to whom becomes harder to judge when you don't have all the pieces of the puzzle. Slang can be decoded; inside jokes may prove harder. "Shaka when the walls fell." If you're a Trekkie, you know. But if you have an inside joke like that within a small group of foreign language speakers language could obfuscate some things. But it may not be enough to render the whole recording useless.

We have come to a point where lenses are so small. With the possibility of deepfakes, I think law enforcement may no longer just send people in with a mic only. Sure, a video can be deepfaked as well but it will be harder to fake and maybe easier to prove that it was.