When viewing Special:ListProperties, all properties shown are rendered in the same way and labelled as the user's language, regardless of whether they're actually in that language or not. For example, with the language set to Gaelic (gd) -
<li><a title="Property:P27" href="/wiki/Property:P27"><span class="wb-itemlink"><span class="wb-itemlink-label" lang="gd">country of citizenship</span> <span class="wb-itemlink-id">(P27)</span></span></a></li>
<li><a title="Property:P30" href="/wiki/Property:P30"><span class="wb-itemlink"><span class="wb-itemlink-label" lang="gd">mòr-roinn</span> <span class="wb-itemlink-id">(P30)</span></span></a></li>
Note that both use lang="gd", although P27 is using the English (en) label, and P30 is using the correct Gaelic (gd) label.
When seen elsewhere, eg on an item, fallback labels are correctly shown as lang="en":
<div class="wikibase-statementgroupview-property-label" dir="auto"><a title="Property:P27" href="/wiki/Property:P27" lang="en">country of citizenship</a><sup class="wb-language-fallback-indicator">Beurla</sup></div>
Patches for review: