Editorial: fix lang description (#1133)

index.html

@@ -547,10 +547,14 @@ <h3>
         </h3>
         <p>
           The [=manifest's=] <code><dfn data-export="" data-dfn-for=
-          "manifest">lang</dfn></code> member is a <a>string</a> in the form of
-          a <a>language tag</a> that specifies the primary language for the
-          values of the manifest's <a>localizable members</a> (as knowing the
-          language can also help with directionality).
+          "manifest">lang</dfn></code> member is a [=string=] in the form of a
+          [=language tag=] that specifies the language for the values of the
+          manifest's [=localizable members=].
+        </p>
+        <p class="note">
+          Specifying the language can improve the user experience by helping
+          user agents to select appropriate fonts for displaying manifest
+          values or to choose the right text-to-speech voice for accessibility.
         </p>
         <p>
           A <dfn>language tag</dfn> is a <a>string</a> that matches the