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TRANSCRIPT, 39 bytes

End is here. >End, Hello, World! >X End

Here's a nice and simple one.

First safe cop! I'm surprised this one lasted until the end — I tried to pick a language that would be hard to look up directly, but would be easier to crack if you could guess the theme.

TRANSCRIPT is an esolang based on interactive fiction games. It has NPCs (strings) and objects (integers). Here End is the name of an NPC.

The first line declares the NPC with the syntax <name> is here. . The second line then assigns the NPC the string "Hello, World!" , and the third line prints the string using the X / EXAMINE command. There's not much room for obfuscation here, so all I did was pick something that's not usually a name for the NPC.

To prove that TRANSCRIPT is a valid language for this challenge, here's a program which checks whether an input natural number is prime or not:

The Nineteenth Room In the middle of the room you spot a lone figure. Martin is here. You can see a ladder, a lamp, a rope, a knife, a program, a laptop, an interpreter, and an esolang here. >RESTORE Which save file would you like to restore? >PROGRAM.sav Done. >SET LAMP TO 1 You turn on the lamp. >LIFT KNIFE You pick up the knife, feeling powerful. >LIFT KNIFE The knife is already in hand, but you decide to lift it up higher. You know knives aren't dumbbells, right? >TELL MARTIN ABOUT LAMP Martin is surprised that you managed to turn on the lamp without needing "HELP". >HELP Too bad, no hints for you. >SHOW KNIFE TO MARTIN You pull out the knife. Martin picks up his phone and starts calling for the police. You quickly realise your mistake and apologise profusely. Good job. >ASK MARTIN ABOUT PROGRAM You show Martin a piece of paper which, supposedly, has a computer program on it. The program appears to be written in a strange and foreign language. Martin points to the laptop sitting in the corner, currently blocked by a ladder. >LIFT LADDER You move the ladder slightly out of the way. >SHOW PROGRAM TO MARTIN Martin doesn't respond. He's too busy trying to golf esolang quines. >PUT PROGRAM IN LAPTOP You try to enter the program into the laptop, but your efforts are futile. The laptop is off. >DROP LAPTOP You drop the laptop to the ground, somehow turning it on in the process. Just kidding, it's still off. The screen has an extra crack now though. >ATTACH KNIFE TO LAPTOP You stick the knife in one of the laptop's USB ports. The laptop turns on. >SET ROPE TO 0 You grab both ends of the rope and tie a knot, forming a loop. >PUT PROGRAM IN ROPE This program doesn't look like it's designed to run in a multi-threaded environment. >CUT ROPE WITH KNIFE The knife is powering the laptop. >HIT ROPE WITH KNIFE The knife is still (somehow) powering the laptop. >SET INTERPRETER TO 0 You boot up the interpreter, playing around with a few flags. >PUT PROGRAM IN INTERPRETER You enter the program into the interpreter. >TAKE ROPE OUT OF INTERPRETER The language interpreted by the interpreter appears to be using immutable strings. >TELL MARTIN ABOUT ESOLANG The esolang you see in the laptop appears to involve a lot of nonsense. >SHOW INTERPRETER TO MARTIN You show Martin the output of the program. It says: "Hello, World!" >ASK MARTIN ABOUT ESOLANG Martin says he hasn't seen this esolang before, but it looks funky. You get so excited about this new esolang that you knock over the ladder. >LIFT LADDER You pick the ladder up and move it a bit further away. >SHOW ESOLANG TO MARTIN Martin tries to study the language. >DETACH KNIFE FROM LAPTOP You pull the knife out from the laptop. The laptop turns off. >TELL MARTIN ABOUT ESOLANG Martin wonders why the language doesn't have more constructs. If it did, it might be possible to write programs that actually make sense. >SHOW LADDER TO MARTIN Martin argues that it's actually a stepladder. >ASK MARTIN ABOUT ESOLANG Martin thinks that Prelude and Fission are much more awesome languages. >MARTIN, Your number was prime. Martin raises an eyebrow, wondering what you're on about. >SHOW ESOLANG TO MARTIN Martin shows *you* Prelude. It is indeed more awesome. >TELL MARTIN ABOUT LAMP Martin already knows about the lamp, remember? >SHOW LADDER TO MARTIN It's a stepladder. >ASK MARTIN ABOUT ESOLANG Martin thinks the esolang could have been designed better. It's fun to write, though. >MARTIN, Your number was not prime. You say this to Martin, but the message isn't intended for Martin. Martin seems to realise. >SHOW ESOLANG TO MARTIN The esolang seems to be called "TRANSCRIPT". >EXAMINE MARTIN It's rude to stare at people like that. >EXIT Thank goodness this charade is over.

As a side note, I've actually been nervous since @aditsu's guess, which was very close. Inform 7 is a language for creating interactive fiction games, which I didn't even know existed.

As a tribute to aditsu's attempt, I gave Inform 7 a try:

"aditsu's close guess" by Sp3000 The Nineteenth Byte is a room. "abandon all work, ye who enter here —aditsu" The laptop is a device in the Nineteenth Byte. A llama is here. Carry out switching on the laptop: say "Hello, World!"

And here's a sample run: