Links to other-language Wikipedias should have an hreflang attribute, indicating the language of the link target. Furthermore, the standard interwiki margin links should have a lang attribute, indicating that the link text is in another language.
In both cases, the content of the attributes should be the same language code as in the Wikipedia's name. Example: links to German wikipedia (de.wikipedia.org) should have hreflang="de", and the interwiki links should look like <a hreflang="de" lang="de" href="http://de.wikipedia.org/...">...</a>.
This feature would aid current or future user agents, including screen readers for the disabled. It could also be used for research projects, and to allow style sheets to style other-language links based on these attributes.
This may be a duplicate of T3433, but I couldn't tell because the examples there don't seem to work.