Since it uses the English encounter name, any other locale will simply fail to register that the boss has already been defeated. The addon will still show "undefeated" unless a localised name is added for each encounter that uses the BOSS method and relies on lockout/tooltip scanning.

If there's a better way to get this data without relying on tooltip scanning, this would be easily solved. The detailed lockout detection could hopefully help with this, once it is finished.

Otherwise, an ugly workaround would be to add all the localised names from wowhead. Ugh...

This has come up once more in Discord. A brief investigation points to ScanInstanceLocks(), where tooltip scanning is employed to compare the text displayed in Blizzard's UI with the hardcoded (English) names from the item database.

That's really a huge hack; maybe there's been some API changes that would make it easier to get at the lockout details in a locale-independent way?

Bumping up the priority since this effectively means all lockout-based defeat detection is broken for any client locale that isn't English.