Can hacked language markers affect Masuda?

Does a Ditto from an American cartridge with a JPN language marker still work with Masuda method? I don’t want to trade for one if people can just make JPN Ditto that won’t work.

I’ve been trying forever to get my shiny Rockruff but I’m way over odds. Did I trade for a garbage fake JPN ditto? Or am I just really unlucky?

The Masuda method involves breeding two Pokémon created in games of different languages, and as long as that Ditto has JPN tag the game will accept it as Japanese, even if it's legit or not.

According to Bulbapedia:

An internal marker on each Pokémon identifies their "original language". If the game recognizes the two Pokémon in the daycare as having different languages, then when an Egg is generated, the game will generate extra personality values in an attempt to find one that results in a Shiny Pokémon.

So your Ditto is suitable, imho. Also because quite every 6IV Ditto out there is a hacked one.

You may just be unlucky, I hatched hundreds of eggs obtained from my Japanese Ditto in Pokemon Y, and I never managed to get a shiny.