When a translation unit is not available in the current page language, it is wrapped with e.g. <span lang="en" dir="ltr" class="mw-content-ltr">Not translated yet.</span> as a way to specify the language that is actually being included there.

The same wrapper is applied when the page is transcluded elsewhere, but not if that page is subsequently transcluded — leaving untranslated units to be indistinguishable from translated ones.

For example create the following two pages:

1. [[Template:Langest]]:

<noinclude><languages /></noinclude>
<translate><!--T:1--> Translated in Template:Langest.</translate>
<translate><!--T:2--> Not translated in Template:Langest.</translate>

2. [[Langtest]]:

<noinclude><languages /></noinclude>
<translate><!--T:1--> Translated in Langest.</translate>
<translate><!--T:2--> Not translated in Langest.</translate>

Translate both into another language (e.g. fr), but only translate the first message of each. The resulting HTML in [[Langtest/fr]] is:

<p>Translated to fr in Langest.
<span lang="en" dir="ltr" class="mw-content-ltr">Not translated in Langest.</span>
</p>
<p>Translated to fr in Template:Langest.
<span lang="en" dir="ltr" class="mw-content-ltr">Not translated in Template:Langest.</span>
</p>

This is correct, and wraps the untranslated units with lang="".

Now transclude that page in a third page, e.g. [[Langtest2]], with {{:Langtest}}, and the resulting HTML is:

<p>Translated in Langest.
Not translated in Langest.
</p>
<p>Translated in Template:Langest.
Not translated in Template:Langest.
</p>

i.e. there is no wrapper on the untranslated content.