The Translate extensions wraps unntranslated headings with the following HTML:

<div lang="en" dir="ltr" class="mw-content-ltr">
== Headline ==
</div>

The parser identifies just the == Headline == part as a heading, so the language markup isn’t present in the TOC, which causes the same issues as what syntax version 2 solved outside of the TOC (correct pronunciation by screen readers, avoiding bidirecionality issues, using appropriate fonts etc.).