Read there ...
http://id3.org/id3v2.3.0/#Unsychronised_ly...t_transcription
http://en.wikipedia.org/wiki/ISO_639
The language identifier can be a string defined by ISO 639 ("deu", "eng", "fra" and so on).
"xxx" is the undefined language identifier string.
Mp3tag uses the "|" pipe symbol as a visible surrogate character for the invisible binary null value.
Therefore Mp3tag is able to support the USLT structure ...
Language|Content descriptor|Lyrics text
Note: There can be more than one USLT frame in a ID3v2 tag, but only one with the same language and content descriptor.
For interoperability between tagger or player applications you should verify whether the other applications do follow the ID3v2 standard or not.
DD.20140728.1030.CEST