HTML Missing lang attributes for websites

The [website](https://this-week-in-rust.org/) is missing a language attribute that prevents the Firefox offline translation tool from being easily used.

https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Global_attributes/lang
https://support.mozilla.org/en-US/kb/how-add-translate-feature-firefox
https://www.firefox.com/en-US/features/translate/

I hope you can optimize it.