In bug 1039069 a dialog got added that allows the user to spoof the locale, which web content/servers are seeing, to en-US if non-en-US builds are used and resist fingerprinting mode is enabled.
That works and the dialog looks properly localized in the old ESR (ESR 60 that is), but in the new ESR (ESR 68) the dialog buttons are shown in en-US while the remaining text is properly localized.
Two crucial pieces the reproduce this bug:
- It only happens during first start
- It only happens if one gets a localized build by using the language packs. The bundles as Mozilla ships them are not affected.
I suspect there is a change that lead to the lang pack .xpis being not early enough available to be used for the dialog button localization on first start.
(Localization might be the wrong component to file this bug in. Sorry, if so but it's hard to find the right one as the cause of the bug is not tracked down yet)