Adding any wrapper elements around headings in MediaWiki interferes with its handling of sections, and the <div class="mw-translate-fuzzy"> wrappers for outdated content – and <div lang="en" dir="ltr" class="mw-content-ltr"> for untranslated content – are no exception. The issues aren't too serious, but seemed worth writing up.
I set up a test page with a fuzzy heading: https://test.wikipedia.org/wiki/Translate_headings/pl
It looks okay with the default skin and settings, but there are bugs in other cases:
1. With the old Vector skin (or any other skin that uses TOC inside the body), the TOC is placed directly before the first heading, inside the fuzzy marker: https://test.wikipedia.org/w/index.php?title=Translate_headings/pl&useparsoid=0&useskin=vector
For some reason this doesn't happen with Parsoid, but I think that's just a lucky coincidence.
2. With Parsoid, the section wrappers are all messed up, marking up multiple uneditable sections and pseudo-sections (https://www.mediawiki.org/wiki/Parsing/Notes/Section_Wrapping#Pseudo-sections) indicated by data-mw-section-id="-1" and data-mw-section-id="-2". https://test.wikipedia.org/w/index.php?title=Translate_headings/pl&useparsoid=1
This is not a problem right now, since section editing for translation pages is not available anyway, but it could cause problems in the future with any tool trying to make sense of the section wrapping.
<section data-mw-section-id="-1" id="mwAQ"><p id="mwAg"><link typeof="mw:Extension/languages" about="#mwt2" data-mw="{"name":"languages","attrs":{}}" id="mwAw"></p> <div lang="en" dir="ltr" class="mw-content-ltr" id="mwBA"> <p id="mwBQ">Lede</p> </div> <p id="mwBg"><span id="A_2"></span></p> <meta property="mw:PageProp/toc" data-mw="{"autoGenerated":true}" id="mwBw"></section><section data-mw-section-id="-2" id="mwCA"><div class="mw-translate-fuzzy" id="mwCQ"> <section data-mw-section-id="-1" id="mwCg"><h2 id="A_pl">A pl</h2> </section></div> <p id="mwCw"><span id="B"></span></p> </section><section data-mw-section-id="2" id="mwDA"><h2 id="B_pl">B pl</h2> <p id="mwDQ"><span id="C"></span></p> </section><section data-mw-section-id="-1" id="mwDg"><h2 id="C_pl">C pl</h2> </section><section data-mw-section-id="-1" id="mwDw"><div lang="en" dir="ltr" class="mw-content-ltr" id="mwEA"> <section data-mw-section-id="-1" id="mwEQ"><h2 id="D">D</h2> </section></div> </section><section data-mw-section-id="-2" id="mwEg"><div lang="en" dir="ltr" class="mw-content-ltr" id="mwEw"> <section data-mw-section-id="-1" id="mwFA"><h2 id="E">E</h2> </section></div></section>
3. On the mobile site, the sections are not collapsible. The untranslated sections have the same problem. https://test.m.wikipedia.org/wiki/Translate_headings/pl
(4. I also remember that it used to cause issues with TOC highlighting on Vector 2022, but I can't reproduce that now – either it's fixed or a more complex page is needed.)
Most of these problems would probably be fixed by placing the markup inside the <h2> etc. instead.