We've noticed there is a large number of users that are having problems changing their locale's language from English. A new package of jomCalendar has been released just now with an updated language file for all of the months and a system plugin has been written to perform the translations as a work-around.
If you would like to translate the language now and currently have the component installed:
1. Download the new package.
2. Extract and upload the new language file and perform the translation.
3. Download, install and enable the system plugin.
If you need to translate it and have downloaded the component AFTER this post, you can simply modify the necessary strings in the language file and install and enable the system plugin.
Kindest regards,