fallback

Thanks for illustrating the explanation so well. But this is not a technical IT interface presentation issue. It's not. It is a political issue.

What the repeated questions and queries about this issue, month after month after month, should be telling us is that many Ukranians would actually prefer the interface to fallback to English rather than to what "readers [...] are more likely to understand", even if Russian is widely spoken in the Ukraine.

What they are repeatedly requesting, month after month, is that they'd like to choose themselves their fallback.

What they're telling you is that they DO NOT want to fallback to Russian, irrespective of whether they're most likely to understand it or not.

Does it take a Phd in Psychology to get this?

One way to get it would be to adopt chinese as MediaWiki's default and then fallback English to German because they're both germanic languages, see how many hairs that would raise...