Language drop-down for a new toot is not marked up for Language of Parts

[ChasBelov]

Steps to reproduce the problem

1. Visit ohai.social in English
2. Click in the What's on your mind? field.
   Type:

The following should be translated as "Did you have boba?"

有冇波霸奶茶?

The following would reasonably be colloquially translated as "No, and I'm miserable!"

冇囉!

3. Select 有冇波霸奶茶?
4. In the UI footer, click EN
5. In the search box which pops up, type Chinese
6. Right-click 中文

Expected behaviour

The tag enclosing 中文 has a lang="ZH" attribute

<div role="option" tabindex="0" data-index="zh" class="language-dropdown__dropdown__results__item" aria-selected="false">
<span class="language-dropdown__dropdown__results__item__native-name" lang="zh">中文</span>
<span class="language-dropdown__dropdown__results__item__common-name">(Chinese)</span>
</div>

The lang attribute must be at the span and not the div level, since the word "Chinese" is in English.

Actual behaviour

The tag enclosing 中文 is missing a lang attribute

<div role="option" tabindex="0" data-index="zh" class="language-dropdown__dropdown__results__item" aria-selected="false">
<span class="language-dropdown__dropdown__results__item__native-name">中文</span>
<span class="language-dropdown__dropdown__results__item__common-name">(Chinese)</span>
</div>

Detailed description

WCAG 2.1 has 3.1.2 Language of Parts (Level AA), which states that all content not in the main page's language must be marked with a language attribute. This is not happening for the language drop-down; the other-language parts are nto marked for language.

Specifications

Mastodon 4.1.0
macOS 13.2.1
Firefox 110.0