@machniak: not sure how to handle translateable strings.

• Do I need to do more to add a new one than adding it to the en_US version?
• should I remove the obsolete $labels['tabattachments'] = 'Attachments'; from all files in localization ?

There's already "More" label in Roundcube core. So, you can just use 'more' instead of 'calendar.tabmore' in the template.