Set `lang` attribute on HTML to current lang

Part of #1106 (closed) (read this issue for access to the full audit and recommendations).

Audit section (Section 3.1.2):

The language attribute of the pages is set to English (lang=”en”) regardless of what language has been selected. Because not everything on the page is translated (yet) this is fine, but as in the future the aim might be to provide a whole page in the selected language, it would be good to already start using the selected language in the lang-attribute.

In case it would stay this way, with some texts in English and some in the selected language, a language change (by means of a different language attribute for those texts) must be indicated for the texts that are different from the language indicated by the language attribute.