Set the lang attribute in the html element to be the right
language, and also add lang attributes to text in a language that
is different to the page language, such as the language switcher
links.
This ensures that links work correctly in languages other than
English. For example, clicking on the navbar "brand" link used to
take you to the English home page even if you were browsing the
site in another language. Now it takes you to the home page for
that language.
