The language code from the header of IDv2 tags and the field UNSYNCEDLYRICS is shown as content in Mp3tag, like eng||. This has no influence on other software that shows UNSYNCEDLYRICS stripped without language code.
This feature allows "replace" actions to be applied to the language code - for instance, changing it from "eng" to "deu" or "xxx" - including in batch mode.
However, a current side effect of this solution is that when copying tags from MP3 files to formats that do not support language codes (such as FLAC), the "eng||" prefix is copied into the lyrics text itself, causing it to be displayed by other software. Removing this text from the target files requires additional manual work.
Perhaps a the behavior could be changed in a way to filter out the header text from the UNSYNCEDLYRICS field before writing when pasting to file formats that do not support such headers?