Currently when processMessageChanges runs with safe imports, messages in other languages that do not have a message definition are treated as safe and imported.
This is also causing messages in message group synchronization cache to time out.
Do not import translations without definitions. Such import should not be treated as "safe-import".
List them on Special:ManageMessageGroups, but add a warning (similar to other warnings we have for different conditions). Maybe also shade them more grey.