cannot edit plugin php file

i am trying to edit wp-favorite-posts.php file so i can change the wordings to a foreign language. i want “Add to favorites” changed to “Legg til favoritter”. when i edit the file and save it, i get this error when i click the add/remove button:

PHP Warning: Cannot modify header information – headers already sent by (output…..

any other way to edit the text?

https://www.ads-software.com/extend/plugins/wp-favorite-posts/