Heya!
I implemented this by making some small changes to roundcubemail/program/include/rcmail.php to load a separate timezones.inc file from the language's localization directory. I also moved the names of the timezones from roundcubemail/program/steps/settings/func.inc to an initial timezones.inc for en_US, and made it load them from rcube_label() instead.
I also added timezones.inc files for all the other localizations with just an empty array, to prevent include errors.
Patch is attached. Criticism welcomed.
Best wishes, Frank van den Brink
P.S. I tried using trac, but its registration page told me that "No handler matched request to /register".
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