text should have a language attribute
It is a very common thing to browse websites some of which are in one's own language, and others are in english, typically. Unfortunately the screen reader and the speech synthesis would have to guess which language the text is, and that cannot always be accurate, in practice this is not actually implemented.
What users then resort to is having keyboard shortcuts for switching between different languages, but that's really not convenient, while the webbrowser does have the language information from the webserver itself, and only lacks a way to transmit that information.
Edited by Samuel Thibault