Going from markdown to epub3, how do I set the language on the HTML tag in the epub?

For epub3, the template looks like this:

<html xmlns="http://www.w3.org/1999/xhtml" xmlns:epub="http://www.idpf.org/2007/ops"$if(lang)$ xml:lang="$lang$"$endif$>

In my YAML metadata block, I have this:

language: ja

This sets the language in content.opf:

<dc:language>ja</dc:language>

But it doesn't set the language on the HTML tag.

I also tried adding

language: ja

to the metadata, but this didn't change the value on the HTML tag, either.