After some dev discussion, we're thinking that the nicest balance would be a PHP variable like "community configuration defaults", which could be set up for eg. an entire project (wikinews, etc.) or for a third-party site, and would be respected as the default values for Community Configuration dialogs, or it would be used directly in the case the the extension is unavailable. Let's check with the Growth team about this.

In T381465#10431829, @awight wrote:

After some dev discussion, we're thinking that the nicest balance would be a PHP variable like "community configuration defaults", which could be set up for eg. an entire project (wikinews, etc.) or for a third-party site, and would be respected as the default values for Community Configuration dialogs, or it would be used directly in the case the the extension is unavailable. Let's check with the Growth team about this.

I guess we need to answer two questions here:

1. What to do if CommunityConfiguration is not installed.
2. What to default to when CLDR is not installed.

1. Is the simpler one IMO. In general we always fallback on a default locale an most sense makes the content language. It would make sense if this at least could be overwritten by a config var in the LocalSettings.php. - I'm not sure if the CommunityConfiguration should take precedence over that one or if it also is like a "kill switch".
2. That's more tricky. The default fallback will be generating a list out of a-z.

Should that be customizable? Maybe at least via $wg setting? In the form of

$wgCite.... = [ 'a', 'b', 'c', ... ];

A hidden backdoor for the numeric system then?