Currently I'm working on folders list sorting and see locale issue. In roundcube we've got setlocale(LC_ALL, 'xx') where 'xx' is a selected language. For Polish it's 'pl', but we need 'pl_PL.UTF-8' for proper sorting. I propose to add locale.inc file with that definition in localization folders or just add label 'localename' with specified value. Not sure what will be better.