@tzzo yes it can only be changed through the admin so at the end is useless for a multi-lang site. What you can do is “hardcode” the lang html attribute as suggested in the link that @DavidB sent you:
{{#post}}{{#has tag="de"}}<html lang="de">{{else}}<html lang="en">
{{/has}}{{/post}}
Technically, my understanding is that you cannot set the html lang attribute through a parameter as you needed in your initial post.
Here is another discussion about working with multilang templates (is not exactly what you’re looking for but is another way to work with templates in different languages):