Quick notes for developers:

• The default numbering system for Arabic is the Eastern Arabic numerals (١, ٢, ٣ ...)
• However, a few years ago, Arabic Wikimedia communities decided to override this and use the Western Arabic numerals instead (1, 2, 3)
• This is done by setting wgTranslateNumerals to false in InitialiseSettings.php

# wgTranslateNumerals @{
'wgTranslateNumerals' => [
	'default' => true,
	'arwiki' => false,
	'arwikibooks' => false, // T5442 -ævar
	'arwikiquote' => false,
	'arwiktionary' => false, // T35758
	'arwikisource' => false,
	'arwikiversity' => false,
	'arwikinews' => false,

• The whole formatNum implementation was recently changed in rMWce8d0e9599a84565d53965481d1c163a90c4e6dd by @cscott This is probably what caused this change in behavior.
• Not sure if this bug qualifies as a blocker T263182 so, leaving it to @cscott to decide.

Looking into it. ::formatNum *should* honor $wgTranslateNumerals, but not impossible something regressed here.

PHP's NumberFormatter is translating the digits even before we get to the part of the mediawiki code which is *supposed* to translate digits:

>>> $fmt = new NumberFormatter('ar', NumberFormatter::PATTERN_DECIMAL, "#,##0.###");

=> NumberFormatter {#2557
     locale: "",
     pattern: "#,##0.###",
      …3
   }
>>> $fmt->format(1234)
=> "١٬٢٣٤"

So we'll have to explicitly turn that off when $wgTranslateNumerals is false. Patch in a moment.

Change 640477 had a related patch set uploaded (by C. Scott Ananian; owner: C. Scott Ananian):
[mediawiki/core@master] language: Honor $wgTranslateNumerals, even if PHP does digit translation

https://gerrit.wikimedia.org/r/640477

Change 640487 had a related patch set uploaded (by Brennen Bearnes; owner: C. Scott Ananian):
[mediawiki/core@wmf/1.36.0-wmf.16] language: Honor $wgTranslateNumerals, even if PHP does digit translation

https://gerrit.wikimedia.org/r/640487

Change 640477 merged by jenkins-bot:
[mediawiki/core@master] language: Honor $wgTranslateNumerals, even if PHP does digit translation

https://gerrit.wikimedia.org/r/640477

Change 640487 merged by jenkins-bot:
[mediawiki/core@wmf/1.36.0-wmf.16] language: Honor $wgTranslateNumerals, even if PHP does digit translation

https://gerrit.wikimedia.org/r/640487

Mentioned in SAL (#wikimedia-operations) [2020-11-10T20:27:25Z] <brennen@deploy1001> Started scap: Backport: [[gerrit:640487|language: Honor $wgTranslateNumerals, even if PHP does digit translation(T267614)]] and [[gerrit:640488|Downgrade the severity of the non-numeric argument to formatNum warnings (T267370, T267587)]]

Mentioned in SAL (#wikimedia-operations) [2020-11-10T21:02:06Z] <brennen@deploy1001> Finished scap: Backport: [[gerrit:640487|language: Honor $wgTranslateNumerals, even if PHP does digit translation(T267614)]] and [[gerrit:640488|Downgrade the severity of the non-numeric argument to formatNum warnings (T267370, T267587)]] (duration: 34m 46s)

Followup task in T268632