Correct. And “of course” 
CSS is about style, not content.
Unfortunately, it does exactly zilch. Nothing. Why would be a question for @cgrunenberg or @BLUEFROG.
The HTML generated by the MMD processor in DT is quite rudimentary in that it doesn’t contain any lang setting at all in the html element. It should do, in my opinion
Even that won’t help you. The lang attribute must be set on an element, preferably the html element. But you can’t include a html element in your MD file. Nor any other element that would set the lang correctly for the whole document.
Perhaps the power that be (@cgrunenberg?) could do something about it?
Edit Here’s what you could do (and I admit upfront that it’s at best a nasty workaround, not a solution).
Create a JavaScript file (say “lang.js”), save that somewhere, and add it to the JavaScript part of DT’s markdown setting.
document.addEventListener('DOMContentLoaded', () => {
const htmlEl = document.querySelector('html');
html.lang = 'de';
})
You can also add this code inside a script element at the top of a MD file to see if/how it works.
This will set the lang attribute to “de” in the html element. It does so, however, after the document has finished loading. So, I can’t promise that this will cause the hyphenation to be modified. Give it a try.