I have a nephew, that will turn 6 this year. I'm thinking of getting 3DS/2DS and a Pokemon game for him. The question is how "text heavy" are Pokemon games? I mean he can read, but not a "foreign language" which English would be for him. 

Would a 2DS be a better way to go than a 3DS (for a 6 year old)?

2DS probably, or keep the 3D turned off on the 3DS.

If it were their native language, i'd say a 6-year-old could handle Pokemon, but not a second language.

what language does he speak, theres like 7 or so languages on X and Y

I wasn't super motivated to learn english in elementary school until i got hold of one of the Zelda games. It helped a lot. Food for thought.

He can dabble a bit in English, since he "learned" it in kindergarten and will do so this year in a preschool. 

His native language is Croatian.

For the first question, the new Pokemon game can actually allow you to choose from 7 languages: English, Spanish, French, German, Italian, Korean, and Japanese. So if he can read any of those and you can set him up, he should be okay.

As for question 2, I think it depends if you think he'll take it places/be roudy with it. If you think there's a good chance he wouldn't be able to be careful, go 2DS. Otherwise, a regular 3DS is better IMO.

Very text heavy. It's an RPG, reliant on menus and stats and such rather than just action. If it's not his first language I would probably just get him Super Mario 3D Land or something. And for that young, I'd say 2DS would be the best choice.

Just get a 3DS 2DS with pokemon.

He'll figure it out. At this age, people learn languages like its nothing.

Yeah pokémon is too text heavy for a 6 yearl old unless he is a prodigy. He will be completely lost in the game.

It really depends on his personality though, but the sureway to go is with a 2DS and super mario (probably the 2D ones are better for an intriduction). It will help him develop his response time to stimulus.

If for example he is the quiet and calm type with developed speech, then you can try something a bit more complex like 3D world or even risk pokémon, cause he might be able to figure it out. Chances are thats  not the case though. Actually, it would be safe to get him a 3DS if he was like this aswell.

you should get a more action oriented games, pokemon might frustrate him from time to time when he has to accomplish certain tasks