Technically, this is a "game advantage" as someone who can speak Japanese and type in it now has advantage over people who can speak Japanese but cannot type in it (PC legit/PS2/XBox360).
This is no more an advantage than someone who can speak/read/type in French, German, Spanish, or the Queen's English....

There are also those JP players who can read/speak/type english to some capacity as well... so, by that comparison you would be labeling those "legit" JP players as cheaters.

On this particular issue, being able to type in non-arabic characters for an Eastern language in an NA/EU client is not breaking the game....if anything, it is more leveling the playing field.