Revert "Switch well-formed to valid in I18N text."

This reverts commit 8affe8b.

Sticking with well-formed to not imply additional validation
requirements.

index.html

@@ -5685,7 +5685,7 @@ <h3>Language and Base Direction</h3>
         <p>
 When the language value object is used in place of a string value, the object
 MUST contain a `@value` property whose value is a string, and SHOULD contain a
-`@language` property whose value is a string containing a valid
+`@language` property whose value is a string containing a well-formed
 `Language-Tag` as defined by [[BCP47]], and MAY contain a `@direction` property
 whose value is a <a>base direction</a> string defined by the `@direction` attribute in
 [[JSON-LD]]. The language value object MUST NOT include any other keys beyond