Splitting from T278253#6939120. If you visit the AbuseLog on frwiki, the following error will sometimes appear in the console:
Uncaught TypeError: Cannot read property 'ownerDocument' of undefined at Object.OO.ui.Element.static.getDocument at Object.OO.ui.Element.static.getClosestScrollableContainer at OoUiPopupWidget.OO.ui.mixin.FloatableElement.togglePositioning at OoUiPopupWidget.OO.ui.PopupWidget.toggle at showTipsy https://fr.wikipedia.org/w/load.php?lang=it&modules=ext.centralNotice.geoIP%7Cext.centralauth.ForeignApi%7Cext.centralauth.centralautologin.clearcookie%7Cext.cx.eventlogging.campaigns%7Cext.echo.api%2Cinit%7Cext.eventLogging%2CnavigationTiming%2CwikimediaEvents%7Cext.uls.common%2Ccompactlinks%2Cinterface%2Cpreferences%2Cwebfonts%7Cjquery%2Coojs%2Coojs-ui-core%2Coojs-ui-widgets%2Csite%7Cjquery.client%2Ccookie%2ClengthLimit%2CmakeCollapsible%2CtextSelection%7Cjquery.uls.data%7Cmediawiki.ForeignApi%2CString%2CTitle%2CUri%2Capi%2Cbase%2Ccldr%2Ccookie%2Cexperiments%2Chtmlform%2CjqueryMsg%2Clanguage%2Cstorage%2Cuser%2Cutil%2Cwidgets%7Cmediawiki.ForeignApi.core%7Cmediawiki.editfont.styles%7Cmediawiki.htmlform.ooui%7Cmediawiki.libs.pluralruleparser%7Cmediawiki.page.ready%7Cmediawiki.widgets.UserInputWidget%2Cdatetime%7Coojs-ui-widgets.icons%7Coojs-ui.styles.icons-content%2Cicons-moderation%2Cicons-movement%7Cskins.vector.js%7Cuser.defaults&skin=vector&version=cl315:97
which seems to originate from ULS (ext.uls.interface.js).
In order to trigger the bug more reliably, you might try adding uselang=<insert-any-code-here> to the URL. When doing so for the first time for a given language, I consistently get the error.
Outcome
Language undo tooltip now works on pages without language settings element and does not cause a JavaScript error.