For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
Please see our announcement here.
function
languages
en-GB
en-GB.function.ini
PLG_STUFF_SOMETHING_LABEL="Something"
PLG_STUFF_SOMETHING_DESC="You use Something to achieve a specific function."
http://fabrikar.com/forums/index.php?wiki/translations/
Language strings for plugins are e.g. in plugins/fabrik_element/fileupload/language