Translate extension interface incorrectly tries to push user to write <br /> instead of <br>, preventing them from editing (at least in TUX). Even if developers of Translate extension prefer the stricter version in line with XHTML, MediaWiki doesn’t differentiate between both, so it shouldn’t be a breaking bug for the extension. W3C specification doesn’t require trailing slash for void elements: https://www.w3.org/TR/html5/syntax.html#start-tags
Then, if the element is one of the void elements, or if the element is a foreign element, then there may be a single "/" (U+002F) character. This character has no effect on void elements, but on foreign elements it marks the start tag as self-closing.
Text: Please replace the following tag with correct ones: <br> → <br />
Point of reference: T134423.