I am trying to localize the plugin into Chinese myself. Over all it is quite pleasant to translate because of the i18n mechanism. Occasionally I found that a certain date format string is not in the PO table. For example, the current month shown in the Month View has date format "M Y", but I cannot find it in the PO table, and the Chinese speakers happen to put year before month. Many other date formatting string are missing, such as the texts in the navigation buttons and the calendar icons in Agenda.
Also, it would be nice to include custom translation entries for "January-December" and "Jan-Dec", in addition to what WordPress provides. Sometimes the WordPress official string does not fit quite well in the calendar view, but changing those strings may affect everything else.
These are some of my humble opinions.