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
See Announcement
We have resolved the issue with the J! updater and this will be fixed in the next release.
Not sure what you mean by "the multilanguage". There are many ways of approaching multilingual sites.Is possible to have the multilanguage in different tables (the same table for every language) without creating manually the tables and the lists into tables for every language?