I'd guess a standardized codepage marker like a "start of CP[932]” ｉｓ ｇｏｉｎｇ ｔｏ ｂｅ ｎｅｃｅｓｓａｒｙ CP[1252] at each CP switches but it might be just a necessity. Han unification is a well known problem to Far Eastern but Unicode normalization problem is basically the same as that.