There is JavaDoc of a normal Java API with an English edition and a Chinese edition, but it seems to require having separate source code for each edition. Is there any more convenient way to do this?
No, there is essentially no way of doing this.
The only work-around that comes to mind applies to the generated HTML pages: you could surround the JavaDocs in block elements that toggle between languages using CSS. Consider:
/**
* <div class="en">Documentation in English</div>
* <div class="nl">Documentatie in Nederlands</div>
*/
public void myFunction() {}
Subsequently edit the CSS of the JavaDocs so that the user can switch languages, e.g.:
div.en { display:none; }
div.nl { display:block; }