One way to do it would be to extend the build-i18n script to copy over translations from fallback languages for the messages where the translation for the main language is not available. This avoids having to query for all messages from multiple languages.

@Iniquity Is there a list of languages and their fallbacks available somewhere?

In T253309#6988625, @SD0001 wrote:

@Iniquity Is there a list of languages and their fallbacks available somewhere?

https://phabricator.wikimedia.org/source/mediawiki/browse/master/languages/messages/?grep=fallback%20%3D I think this is the list :)
+ plain text https://phabricator.wikimedia.org/T258492#6490574

This is addressed by this and this commits, should be resolved when the change goes live. The only issue is that configuration files on wikis don't know which i18n files exist and what fallbacks are, so they firstly request the main file and the main file requests fallback i18n. (But when there is a configuration file on a wiki, there is usually i18n also. Another possible case is when a language without i18n is used as an interface language.)