Rename tag by folder name

My library is organized in folders and subfolders but some album name tags are missing, is it possible to rename the tag with the same name as the folder the file is?

Yes, use the menu option "convert > filename to tag" Then you can use masks to properly take the info in the directory/filename to fill in the album tag value. See here:

http://help.mp3tag.de/main_converter.html#ftt