* (bug 4901) lang, hreflang attributes added to sidebar interlanguage links

* (bug 4901) lang, hreflang attributes added to sidebar interlanguage links

The lang attribute assists screen readers in reading out the language name correctly; see sample recordings from iOS's VoiceOver reader mode attached on <https://bugzilla.wikimedia.org/show_bug.cgi?id=4901>
hreflang attribute is less clearly useful, but may assist some tools in indicating the language of the destination site.

Inline interwiki links have not been changed.

Release notes will be added on 1.18 channel, this is an old bug fix and easy to backport.