Reuven Nisser <rnisser@ofek-liyladenu.org.il>:
>
> 1. The lang attribute set for the element itself.
> 2. The closest parent element that has the lang attribute set (i.e., the
> lang attribute is inherited).
> 3. The HTTP "Content-Language" header (which may be configured in a
server).
> 4. User agent default values and user preferences.

Why exactly is 2. or 1. not an option for you?

> 1. What is the language of "3"? Should it be spoken in Hebrew or English?
> 2. What is the language of "I have 3 brothers".
>
> What do you think about my solution?

That it's not really one, as you demonstrated yourself above there's still a
need to mark up the parts of the document.