Microsoft’s search engine has a different way of establishing a web page’s language than Google. The documented HTML-tag to make BING recognize the language is through the
<meta name="content-language" content="en"></meta>
Typically, Bing will be able to guess the actual language based upon the actual text. But this is not guaranteed to work in all cases.
It would be handy in terms of presenting relevant search results in Bing to also set the
content-language on Discourse.