Rhythmbox is using gstreamer for handling all media files. Currently gstreamer can read ID3v1, ID3v2.3 and ID3v2.4 but can only write ID3v1 and ID3v2.4. As it was described in the previous post ID3v1 is no good for other characters than English on the other hand ID3v2.4 is not supported by all hardware players (mine doesn't). The most portable solution is to use ID3v2.3 which doesn't have write support out of the box in gstreamer.
If you still have your original content you can try to re-encode them with sound-juicer. It's also possible to re-encode the files with a shell script, try to see if the following command will add proper ID3v2.4 tags:
Replace original.mp3 with one of your MP3s and copy.mp3 with the new file name. WARNING: DO NOT use the same file name for the original and the copy otherwise you will lose your MP3.
gst-launch filesrc location=original.mp3 ! id3demux ! id3v2mux ! filesink location=copy.mp3
For better portability with hardware players, there's a plugin in gstreamer's bugzilla that adds support for writing ID3v2.3 tags. The source code of the plugin, the build instructions and the user manual are in this bug:
Download the .tar.gz file and follow the instructions in the file README.txt.