I am just supposing, but have not checked, that if they change the text they must also change the DOM by at least changing the lang attribute on nodes affected,

I am just supposing, but have not checked, that if they change the text they must also change the DOM by at least changing the lang attribute on nodes affected, meaning probably the lang attribute on the html element, but could also be a lang attribute on each element wrapping a textNode. on edit: I figured the article must have said something about this and I missed it, and yes, it shows that the DOM is changed to be lang="nl" on the html element, which means obviously if React rerenders but does not rerender the HTML element (which many React applications do not control) then the language would be out of sync, of course.