You asked whether or not the language code mul (multiple languages) should be mentioned.  I felt that it should since it is out there.  As for recommending it, normally, I do not think mul is a good language tag, since it is not specific enough to be useful.
However, I have a document I transcribed from an image of a Renaissance French (different than either modern or medieval French; thus no language tag is available); unable to find an experimental tag in use as of yet, I created the experimental tag x-fr-rn  but firstly, I have added summaries of the text in English; secondly experimental tags may not be that useful .  Also I'd like to target both English speakers, French learners who speak English, and possibly native French speakers who would like to see this particular document about the French in North America.   So I've considered using the language tag "mul' somwhere here.  But it's useless as a text-processing language, of coruse.
I note that documents in multiple languages are commonly used to target language learners; if more of sites for learners end up on the web, then these sites should become common not rare.
Section 7, Choosing Language Values, you put the information about the expected updating of "Language Tags in HTML and XML" in an editorial note.  This goes in the draft I think, till the updating occurs; then you change the text to say that this is the updated "Language Tags . . . ", the most recent version of this document,  which has been recently updated.
Would not it be a best practice to specify font whenever the user agents might be unaware of the various language tags as in the case of the Chinese tags, zh-HANS, zh-HANT and thus might choose the wrong font (Best Practice 13 in Section 7, "Choosing Language Values")?
sometimes you state that the metadata about the language of the targeted audience should be stated in the HTTP content-language header (see 3.1, last paragraph) and sometimes you state that it is not determined where it should be stated as the use of the meta tags is becoming more and more widespread (since even some tools for creating pages are sticking these in) (see 4.2).  Then in Best Practice 8 (section 6) you recommend using the meta tag.  So three different recommendations for how to to this--seem to be conflicting to me.
A small note on setting the text processing language in the html element:  the html element is not always available for this purpose--in for example cases where your page is embedded by the host into its page, which is quite common I think (a site that does this with html pages is teacher web, ).  Here your html page, if you choose to have one, is truncated at the body tag so nothing from the body tag or above is embedded into the host's page (usually you format the rest of the page in a div or table; if you want to use css you have to put these in the page you create below the body tag or these are not included in the embedded page!).  So this means that to set a text processing language for the whole page you have to insert your page into a larger div or something where you set the language.  
In your draft, you listed several current implementations that use language information--including:
* detection of language by voice browsers;
* linguistically-sensitive searching;
* serving a version of a page according to language preferences set by the user--sometimes possible
I feel these should all be mentioned from the beginning, in for example, 2, "Why Specify Language," where you state, for example, "voice browsers in the accessibility world" are an existing implementation that uses language information; might as well list all three.
I felt that the titles of best practices in your table of contents, that serve as your headers were vague.  I often preferred the more specific statement just beneath the header.
"Best Oractuce 1:  Always declare language in the html tag."
Below you are much clearer,
Always declare the default text=processing language . . . ."
why not incorporate this into the best practice heading,
"Always declare default text processing language in the html tag" (as I noted above, though, this is not always an option, when one's page is embedded). 
? want more specific word for "applying" (possibilities include "creating the markup for", "displaying", "developing and displaying content with") in the following sentence:
"Specifying the language of content is useful for a wide number of applications, from linguistically sensitive searching to applying language-specific display properties."
1.1  Who Should Use This Document
? want word other than "deployment" (sorry, it gets to me, we have heard it too much here in reference to troops perhaps), in "support for international deployment"
I tried to rewrite this, but it came out too long:
"that support content authors in internationalizing web content (thus making it viable with current and future implementations of processes that rely on language information).
Continuing with the next sentence (which began, "Enabling international deployment . . . "), I rewrote it as:
"As noted above, with more implementations on the web using language information, internationalization has become the responsibility of all content authors . . . "
1.4  "immediately to hand"  you mean "immediately at hand"
3.3  Subject heading "Relationships between language, characer encoding and directionality"
I am a novice to this terminology I guess, with a background in linguistics, where "directionality" can mean a variety of things, not all of them at all related to the way print is arranged on a page.  So I would have preferred that you used the words, "text direction", or "direction of text" (which you use right below this; but I want it clarified in the heading, I guess)