Language = pa;
Input = string(3) "੫"

Backtrace:
string(1681) "<ul>
<li>Language.php line 2965 calls wfBacktrace()</li>
<li>Language.php line 2896 calls Language->commafy()</li>
<li>Message.php line 617 calls Language->formatNum()</li>
<li>Message.php line 595 calls Message->extractParam()</li>
<li>Message.php line 451 calls Message->replaceParameters()</li>
<li>Message.php line 493 calls Message->toString()</li>
<li>View.php line 1827 calls Message->parse()</li>
<li>View.php line 2122 calls LqtView->getShowReplies()</li>
<li>View.php line 1918 calls LqtView->showThread()</li>
<li>View.php line 2113 calls LqtView->showThreadReplies()</li>
<li>View.php line 1918 calls LqtView->showThread()</li>
<li>View.php line 2113 calls LqtView->showThreadReplies()</li>
<li>View.php line 1918 calls LqtView->showThread()</li>
<li>View.php line 2113 calls LqtView->showThreadReplies()</li>
<li>View.php line 1918 calls LqtView->showThread()</li>
<li>View.php line 2113 calls LqtView->showThreadReplies()</li>
<li>View.php line 1918 calls LqtView->showThread()</li>
<li>View.php line 2113 calls LqtView->showThreadReplies()</li>
<li>TalkpageView.php line 352 calls LqtView->showThread()</li>
<li>Dispatch.php line 63 calls TalkpageView->show()</li>
<li>Dispatch.php line 217 calls LqtDispatch::talkpageMain()</li>
<li>- line - calls LqtDispatch::tryPage()</li>
<li>Hooks.php line 253 calls call_user_func_array()</li>
<li>GlobalFunctions.php line 3815 calls Hooks::run()</li>
<li>Wiki.php line 412 calls wfRunHooks()</li>
<li>Wiki.php line 305 calls MediaWiki->performAction()</li>
<li>Wiki.php line 555 calls MediaWiki->performRequest()</li>
<li>Wiki.php line 448 calls MediaWiki->main()</li>
<li>index.php line 59 calls MediaWiki->run()</li>
</ul>
"

Notice: Undefined offset: 0 in /www/dev.translatewiki.net/w/languages/Language.php on line 2966

I'd say LQT is passing invalid input to wfMessage which calls Language::formatNum

https://gerrit.wikimedia.org/r/37633