List manager uses default joomla language files system. It will load the language selected for front or backend according to your joomla main configuration.
If you have your site configured for 'english (en-GB)', it will load language files from
(front-end) joomla\language\en-GB
(back-end) joomla\administrator\language\en-GB
en-GB.com_listmanager.ini and it will also default to the folder

> if another language is selected but no file translation is found. You can download the some language files other customers have provided from your customer area, maybe they are for a previous version, but you can add any translation if you find any text missing.