#mediawiki-extensions-translate apparently has not configured any group by default who should be able to assign and remove the `translationadmin` group by default. The solution has been to assign on each wiki where the translate extension is enabled the translationadmin permission to be added and removed by, mostly, users in the bureaucrat user group. I suggest that we make that default for wikis using the translate extension via a CommonSettings.php configuration, and then clean InitialiseSettings.php for redundant entries but keeping customization by local wikis which have decided this group to be granted and removed by administrators, for example. This will prevent leftovers, such as in wikimania2018wiki, where no user locally is able to assign this, and as such stewards are required for this.
The other solution would be to add on #mediawiki-extensions-translate a default, but maybe it'd be better to have it migrated to extension.json first (T87985).