Soulknife has all the tools you need. Yes, even for the language thing.
Telepathy - go full mission impossible here and have your linguist backup speaking silently in your head. Could be an NPC you have persuaded to help.
Additional ASI over the normal class baseline. If you want an infiltrator take Actor feat.
Expertise and plenty of it. You want expertise in deception here
Now just add a common potion - potion of comprehension. It’s common - if your DM won’t let you find one then your DM is railroading you away from infiltrating. There are other ways but the potion is the easiest and best
So you understand what is being said. You get instructions on what to say back telepathically and you can imitate the speech of your interpreter (actor feat never says you need to understand it yourself). It’s all ver Mission Impossible which fits a rogue theme like a glove.
But in a white room where the DM is making everything fail it fails. In that white room the task is impossible to all classes at all levels - how anyone thinks that’s a fun way to run a game is rather beyond me.