Node teasers should add 'lang' tag if its language differs from the page language

Now that #867114: Search results should add lang tag if language of search result differs got committed, shouldn't we apply this logic to node teasers as well? For example, the list of nodes on the frontpage?

Use case:
- Add another language to the site, eg Dutch
- Create a few nodes: Dutch, English and undefined
- View the frontpage. No language switching in the HTML. Not good?

Current:

<div typeof="sioc:Item foaf:Document" about="/drupal7/node/3" class="node node-article node-promoted node-teaser contextual-links-region clearfix" id="node-3">
  <h2 datatype="" property="dc:title"><a href="/drupal7/node/3">Nederlands bericht</a></h2>
  <div class="content clearfix">
    <div class="field field-name-body field-type-text-with-summary field-label-hidden">
      <div class="field-items"><div property="content:encoded" class="field-item even">
        <p>Nederlandse tekst</p>
      </div>
    </div>
  </div>
</div>

Should be:

<div typeof="sioc:Item foaf:Document" about="/drupal7/node/3" class="node node-article node-promoted node-teaser contextual-links-region clearfix" id="node-3">
  <h2 datatype="" property="dc:title"><a href="/drupal7/node/3" xml:lang="nl">Nederlands bericht</a></h2>
  <div class="content clearfix" xml:lang="en">
    <div class="field field-name-body field-type-text-with-summary field-label-hidden">
      <div class="field-items">
        <div property="content:encoded" class="field-item even">
          <p>Nederlandse tekst</p>
        </div>
      </div>
    </div>
  </div>
</div>