Complex sentence: What modern scholars call "Galician-Portuguese" was originally the native language of what are now Galicia and Portugal, appears to have been used regularly (and amost exclusively) in the Iberian Peninsula as the language for lyric song.
Split into simpler sentences: What modern scholars call "Galician-Portuguese" was originally the native language of present Galicia and Portugal. It appears to have been used regularly in the Iberian Peninsula--and amost exclusively--as the language for lyric song.