Create a script that can look at the gerrit merge history for Translation updater bot for the prior day and check on-wiki to see if the updates were applied. This could act as a canary for nightly l10nupdate process failures.
If things are working correctly, by 05:00Z or so each day the wiki's should have all of the messages that were merged prior to 03:00Z. If they don't, something is wrong. Ideally messages for MediaWiki itself and all installed extensions would be validated. It should be sufficient to only check a small number of updated messages per component rather than attempting an exhaustive validation. The l10nupdate process shouldn't have a partial failure mode per component.