How does one store text from multiple natural languages, such as Japanese and Chinese, in one XML file which has 'English' as the main language?
XML's "natural" encoding is Unicode UTF-8. This means that you can mix characters from different languages freely within an XML document. You will, however, need a UTF-8 editor.
In a project I'm currently working on we mix English and Japanese text in the same documents with English sections enclosed within <english> </english> tags and Japanese within <japanese> </japanese> tags. The only reason we tag the languages is so that we can choose which to display - it does not make any difference to the XML parser. It is perfectly valid to mix any characters together while using Unicode