apt_mirror error: Execution failed: No alternatives found for 'main/i18n/Translation-ar'

Since upgrading Debusine to a 81b3158b, apt mirroring fails.

https://debusine.freexian.com/freexian/debian/work-request/127398/

Execution failed: No alternatives found for 'main/i18n/Translation-ar'

This quite plausibly correlates to recent refactoring of the task performed by @cjwatson.