I understand that technically when a leap second is added, it is added after midnight UTC, but I'm unclear how the addition is handled in other timezones. For precise reckoning of course (e.g. astronomical ephemeris), it must be added at the same instant (e.g. at 1 AM in France, or 7 PM in in New York), but for many locations doing so could be disruptive and potentially confusing. Even in Britain, the addition at midnight on New Year's Eve could be confusing, since it would either extend the countdown by a second, or require it to be started a second "late".
Are leap seconds in fact added everywhere, in all time zones, at the same UTC instant, or do different time zones (and perhaps even different countries and organizations) account for them at different, more "convenient", times.