All text labels which are used by the extension are defined in INI language files. This is best practice which is recommend by Joomla! team and this approach provides ability to add multi-lingual support to extensions. You can create a new translation or edit existing with help of Localise component or any similar extension which can be found here.
The following language files are used by CalendARI component (all paths are specified for English language, for another language change en-GB language tag to tag of the required language):
Frontend
language/en-GB/en-GB.com_calendari.ini
language/en-GB/en-GB.com_calendari.ajax.ini
Backend
administrator/language/en-GB/en-GB.com_calendari.ini
administrator/language/en-GB/en-GB.com_calendari.ajax.ini
administrator/language/en-GB/en-GB.com_calendari.sys.ini
Language files for modules are located in language/en-GB folder, for plugins in administrator/language/en-GB folder.