Ok now I understand what it is but I still need to have it removed. This is because the automatic lyrics finder foobar200 component that I use handles the songs with language identifiers as if they didn't have a lyrics tag at all for some reason. This way the finder will search for lyrics for files that already have them, lengthening the process too much. The component does recognize the lyrics when they don't have the language ID though.
Transfering the lyrics from UNSYNCEDLYRICS to LYRICS is not an option because the ipod won't read them that way.
What's weird is that I do have some tracks without the language identificator in the UNSYNCEDLYRICS field, and they all look like this:
They all have two UNSYNCEDLYRICS fields, the first ones don't have the language ID, and are the ones displayed in the list view as you can see and the ones that the foobar component is taking into account, because those files are ignored when searching for lyrics. Although those are the only files where that happened and when I tried to do the same with other files, the language ID always came up again.