It only converts it to make it readable. It should be saved as is with the current name and you overwrite the existing file. If it was a .strings file, then you must leave it as a strings file. If it was a plist file, then leave it as a plist file...
Google for pledit. A very simple Windows based plist editor. Easy to find. Change the .strings extension to .plist, edit and save the file, change it back to .strings. You're done.