Template:Wikipedia languages/core
This is the /core template for {{Wikipedia languages}}, which is the template for the "Wikipedia languages" section on the main page.
The template is used to ensure a consistent formatting of the links, which hasn't always been the case (for some time, the Japanese and Chinese links were within {{Lang}}, which categorized the main page within Category:Articles containing Japanese language text and Category:Articles containing Chinese language text.
Technical part
The links of this template show a mouseover, which is the English language name of the language and its IS0 639 code within brackets. To determine the English name, it calls {{Language}}. Furthermore, there is a "lang" attribute and the "xml:lang" attribute is automatically added by MEdiaWiki within the text so that browsers recognize it's a non-English text and treat it properly. The text of the link then uses the parser function {{#language:}}, which displays the language's native name as it's used by MediaWiki, e.g. within the interwiki links. This is done to ensure consistency with the interwiki links in the sidebar.