Note that we couldn't use this syntax because it would prevent you writing mixed language content (you would only see one of the bits of content at a time depending on your language). This is supported by standard HTML (and is useful) and we would be breaking it.
Could you please elaborate on which syntax you're referring to exactly, and what would break? My original suggestion is to keep the syntax exactly the same as it already is in Moodle. We'd just be moving the display processing from the server to the client. Is Moodle doing something I missed in my CSS re-implementation? The only "syntax" change should be that <div class=multilang ... can be used just like <span class=multilang ... Well, there is the secret "secret" bit is that it actually allows more flexibility because you could do things like have the same text for two languages:
<span lang="en af">Jesus</span>
<span lang="es">Jesús</span>