I have an Italian 3DS and I've bought a second-hand English Mystery Dungeon: Explorers of Darkness. I should be able to play it, but I can't, and I don't know why.
According to this question "Can I play DS games from any region on 3DS?":
While all 3DS software is region-locked, Nintendo DS software is not. Because that region-locking isn't encoded into the DS software, you can play software from all regions on your 3DS.
However, software specific to the Nintendo DSi are region-locked, as Wikipedia states.
I thought that maybe that game itself could be region-locked, but the only region-locked Pokemon game should be Pokemon B2W2, according to Serebii:
The games can be played on any model of DS and 3DS, however they contain a region lock on DSi and 3DS models.
I'm pretty sure that Mystery Dungeon is not an enhanced DSi game.
As I've found on Gamefaqs:
There is an easy way to tell if a game is DSi-enhanced or not: look at the letters on the bottom of the label on the cartridge. If it begins in "TWL", it's DSi-enhanced. If it begins in "NTR", it isn't.
And that's not the case.
So, here's my question: is there something I'm missing? Why I can't play that game on my Italian 3DS?
Here is a picture of the cart:
Best Answer
Your research is spot on. Any NTR code DS game will work on any 3DS. So, why doesn't it? The most likely reason is that it's a fake/copy/bootleg. These are quite common, especially for popular brands like Pokémon. If you post pictures of the cart (both sides), box front and back and manual, I might be able to tell you if it is indeed one.
Looking at your specific cart: that looks legit indeed. It's a European copy though, so your 3DS actually has the correct region code anyways.
It might actually be damaged then, even if it works on the DS. You could try cleaning the contacts with a Q-tip and alcohol.