Hmm... Maybe you're either unlucky, or the answer to whether or not the Nature breeding feature works in terms of pokemon outside the game's respective language depends on whether or not the non-ditto pokemon is the same language as the game, and if the latter is true, then the following is true as well:
If the game is an English game, the non-ditto pokemon has to be an English language, non-ditto pokemon in order to breed its Nature. Otherwise, it can't breed its Nature onto the offspring, and the offspring will be translated into English...
But if you're just unlucky, then the above two sentences are not true...