Thank you for your code, you excellent documentation and the help you offer here.
After installing Theme Switcher I have lost the localization of the datepicker. I have copied catalog/includes/languages/english/modules/header_tags/ht_theme_switcher.php to catalog/includes/languages/swedish/modules/header_tags/ht_theme_switcher.php (by the way, it would be a good idea to warn in the documentation that this step is needed when using more languages), and I have checked that I have the line
define('JQUERY_DATEPICKER_I18N_CODE', 'sv');
in both catalog/includes/languages/swedish.php and catalog/admin/includes/languages/swedish.php,
and that i have the filecatalog/ext/jquery/ui/i18n/jquery.ui.datepicker-sv.js with the right settings, but the default English datepicker appears on Swedish pages.