I really like All-in-one Event Calendar, kudos and many thanks to The Seed for keeping it free.
I am testing ai1ec in my two dev multilingual websites (one uses the excellent Polylang plugin while the other is multisite and uses the excellent Multisite Language Switcher plugin). In both of them I have the same problem: if untouched by the user the calendar view is alright in all the languages that I use, but as soon as the user changes view (weekly, agenda...) or clicks on previous/next buttons the language of the view becomes inconsistent and many terms (if not all) fall back to english.
I thought that it might depend on the .po files, wether they are complete or not, therefore I created from scratch an italian it_IT.po file (I am sure that I have translated all the terms that are in the .pot file) but the problem persist.
In the website with Polylang I have also another problem: the calendar views can't keep separated the events by language, no matter what language the user choose the calendar shows the events in all the languages.
Thanks a lot!!!
P.S. The Seed: let me know if you are interested in the italian translation, I could email you the .po .mo files.