You can't mix encodings in a single file. A file has one encoding only. It was not possible before Unicode to mix two languages in a single file, whether the languages involved were Chinese or Japanese or French(English was an exception).

Now, if a file was encoded in Unicode, and/or if it was in such document format that support inline font specification, such as HTML, then you could mix two languages without having to stick to one language by e.g. wrapping <font face=Helvetica>paragraphs and words</font> <font face=Futura>with tags</font>.

My point is, it seems that the author is not aware that each of CJK languages are only understood within each countries, in both writings and speeches, and that's somewhat peculiar.