This version of Crystal takes place after an instance of Pokémon Red and Blue in which the player performed Missingno. glitch.Everyone went completely batshit in the intervening two years as Missing No wandered across the PokčPlanet. This explains all the screwy dialogue, nobody is sane enough to properly understand each other anymore.
This version casts a foreigner to Johto and Kanto as the lead who just got there, not a native.What does this explain? Well, Pokemon trainers would greatly benefit on their travels from understanding more than one language to travel the world and battle foreign opponents, but to someone trying to understand a language for the first time, especially the kind of young lead you play in a Pokemon game, nobody would be any the wiser and would come across like they're reading from a foreign language dictionary with pages ripped out. And since our hero/heroine doesn't bother with asking for corrections, your understanding is obviously less than perfect...