inc/parsers/displayname.php
If the first line for a page starts with #DISPLAYNAME, the rest of the line is used to define the page title, when the page is shown and also the the text of the category links. For normal internal links, it is not used, but you can use the pipe there.
If you use language subpages, only the #DISPLAYNAME of the language subpages is used and not the included page. So you must define it for each language.
The displayname is stored in the swWiki->displayname property.