The problem is that you're solving a single subset of a single UI screen, while the problem can and will happen in many different UIs. The source of the problem is a single scenario where the user has a repackaged Thunderbird and then somehow ends up with a langpack for the same locale. This setup is pointless and should never happen, but it seems that it happens often for some reason. My suggestion is to disable those langpacks.
Bug 1728744 Comment 43 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
The problem is that you're solving a single subset of a single UI screen, while the problem can and will happen in many different UIs. The source of the problem is a single scenario where the user has a repackaged Thunderbird and then somehow ends up with a langpack for the same locale. This setup is pointless and should never happen, but it seems that it happens often for some reason. (the reason it was not a very high priority in Firefox is that in Firefox it does happen very rarely) My suggestion is to disable those langpacks.