Vanilla JavaScript: Is there a way to toggle multiple CSS-classes in one statement?

No it is not possible using Element.classList API directly. Looking at API you can read:

toggle ( String [, force] ) When only one argument is present: Toggle
class value; i.e., if class exists then remove it, if not, then add
it. When a second argument is present: If the second argument is true,
add specified class value, and if it is false, remove it.

Reference here.

You could potentially write your own “utility” function (in vanilla JS) which does what you want, below a very simple demonstrative example which work on top of the classList API:

var superToggle = function(element, class0, class1) {
  element.classList.toggle(class0);
  element.classList.toggle(class1);
}

And you call it in this way:

superToggle(group,'left-to-right', 'right-to-left');