[selectors-4] Defer to css-text for definition of content language so…

… there's only one defining instance. Fixes #897.

selectors-4/Overview.bs

@@ -1849,9 +1849,10 @@ The Directionality Pseudo-class: '':dir()''</h3>
 <h3 id="the-lang-pseudo">
 The Language Pseudo-class: '':lang()''</h3>
 
-	If the document language specifies how the (human) content language of an
-	element is determined, it is possible to write selectors that
-	represent an element based on its language.
+	If the document language specifies how
+	the (human) <a>content language</a> of an element is determined,
+	it is possible to write selectors that
+	represent an element based on its <a>content language</a>.
 	The <dfn id='lang-pseudo'>:lang()</dfn> pseudo-class represents an element that
 	is in one of the languages listed in its argument. It accepts
 	a comma-separated list of one or more <a>language ranges</a> as its
@@ -1860,15 +1861,15 @@ The Language Pseudo-class: '':lang()''</h3>
 	(Language ranges containing asterisks, for example,
 		must be quoted as strings.)
 
-	The <dfn>language</dfn> of an element is defined by the document language.
-	For example, in HTML [[HTML401]], the <a>language</a> is determined by a
+	Note: The <a>content language</a> of an element is defined by the document language.
+	For example, in HTML [[HTML5]], the <a>language</a> is determined by a
 	combination of the <code>lang</code> attribute, information from
 	<a element>meta</a> elements, and possibly also the protocol (e.g.
 	from HTTP headers). XML languages can use the <code>xml:lang</code>
-	attribute to indicate language information for an element.
+	attribute to indicate language information for an element. [[XML10]]
 
-	The element's <a>language</a> matches a <a>language range</a> if
-	the element's <a>language</a> (normalized to BCP 47 syntax if necessary)
+	The element's <a>content language</a> matches a <a>language range</a> if
+	its <a>content language</a> (normalized to BCP 47 syntax if necessary)
 	matches the given <a>language range</a> in an <var>extended filtering</var>
 	operation per [[RFC4647]] <cite>Matching of Language Tags</cite> (section 3.3.2).
 	The matching is performed case-insensitively within the ASCII range.