Just came across some files with no language code in the USLT field just "00 00 00" in hex editor. Maybe this is not allowed? but anyway I guess some software does it. Other programs (e.g. Foobar2000, iTunes, dbPowerAmp) read and write these tags successfully without modifying the language code.
Problem with Mp3tag is when the field is initially read it displays with 2 leading pipe chars e.g.
||My beautiful lyric
When the file is written the pipe chars are interpreted as part of the lyric and it becomes
eng||||My beautiful lyric
Can you make Mp3tag "play nice" with these files also please?