OK... this is no longer happening. I have literally no idea what went on there, but am attaching screenshots of two still-open tabs to verify I haven't gone mad. In the diff, clicking previous diff would show it in English, then clicking next would show it in Polish.

This is currently happening again, this time in Chinese, and the last person who edited has a Chinese username.
https://commons.wikimedia.org/w/index.php?title=Commons:Administrators%27_noticeboard/User_problems&diff=next&oldid=303908227

On the previous diff, where User:廣九直通車 did not sign, the search box is in English.... but I anticipate that this will disappear again, even from the diff (!) once someone else edits the page, as it did last time with Ankry.

... actually this has disappeared without an intervening edit to the page.

Now in German, presumably because User:Sänger is the last editor of the page.

So i get the german search box, and the cache key is:

Saved in parser cache with key commonswiki:pcache:idhash:1067011-0!canonical and timestamp 20180610140040 and revision id 305638763

Which is odd.

So something is using interface language when it should be using canonical language.

<inputbox>
bgcolor           = transparent
type              = fulltext
namespaces        = Commons
prefix            = Commons:Administrators' noticeboard
break             = no
width             = 50
placeholder       = {{LangSwitch|en=Search in all administrators' noticeboard archives|fa=جستجو در همهٔ بایگانی‌های تابلوی اعلانات|sv=Sök i alla arkiv|fi=Etsi ilmoitustaulun arkistosta|ja=全伝言板倉庫検索|ko=모든 관리자 요청 보존문서 찾기|pt=Pesquisar em todos os arquivos|uk=Шукати в усіх архівах дощок для оголошень|zh-hans=搜索所有管理员通知栏档案}}
searchbuttonlabel = <!-- &#x1F50D; not very public unicode -->
</inputbox>

So it looks like its using the default for the search button. So its probably a bug in the extension that generates that label.

yeah,

line 138:
$this->mSearchButtonLabel = wfMessage( 'inputbox-searchfulltext' )->text();

It should be
$this->mSearchButtonLabel = wfMessage( 'inputbox-searchfulltext' )->inContentLanguage()->text();

And ditto for a bunch of other messages

We generally use inContentLanguage() for configuration messages which are not supposed to be translated.

In T194815#4270281, @Nemo_bis wrote:

We generally use inContentLanguage() for configuration messages which are not supposed to be translated.

Well we also use it for things that get embedded in pages since page content is assumed to stay constant for different users (an assumption which admittedly makes about no sense on commons, but its pretty ingrained in mediawiki at this point)

The ideal case is to actually use ->inLanguage() feeding it the language from the ParserOptions object.

Still happening, FWIW

Change 758950 had a related patch set uploaded (by TheDJ; author: TheDJ):

[mediawiki/extensions/InputBox@master] Inputbox labels should be in content language

https://gerrit.wikimedia.org/r/758950