Is there a way to edit one field by using csv import? For example, I want to add "language" in the sublist since I did't set the field language when I first imported csv file... When I tried to import csv with adding "language" column, it created a separate line. If it's not possible do I have to manually edit the wrong imported lines?

when you do a CSV import to add the language to the existing sublist "localization", you need to included a "key field" (presuming this is "accounting context" here, but might be line id) check the mapping page for this sublist, you're looking for a field with the below icon