See ... http://id3.org/id3v2.3.0
As defned by id3.org in the section 4.9 USLT [Unsychronized lyric/text transcription], the USLT frame header can be constructed of five or more characters.
In general it is not harmful to leave the content descriptor empty.
There may be more than one 'Unsynchronised lyrics/text transcription' frame in each tag, but only one with the same language and content descriptor.
See also ...
Adding mp3 comments results in illegal tag?
DD.20140512.1656.CEST