Elisia no, it is a Portuguese-Galician. The Cantigas of Santa Maria are a set of four twenty-seven compositions in Galician-Portuguese, who in the thirteenth century was the fundamental language of lyrical cultured in Castile. Are divided into four manuscripts, one in the National Library of Spain (Codex To, by Toledo), two in the Escorial (Codex E and T) and the fourth in Florence (Codex F).

The music:

Maravillosos et piadosos et mui fremosos miragres faz
Santa Maria, a que nos guia
ben noit' e dia et nos dá paz.

E d'est' un miragre vos contar quero,
que en Frandes aquesta Virgen fez,
madre de Deus, maravillos'et fero, por hua dona que foi hua vez
a sa eigreja d'esta que seja
por nós et vejamola sa faz
no parayso, u Deus dar quiso
goyo et riso a quen lie praz.

Maravillosos ...

A questa dona leuou un menynno,
seu fillo, sigo, que en offrecon
deu aa Virgen, mui pequenynno,
que de mal ll'o guardass'e d'oqueijon
et lle fezesse per que disesse
sempr'e soubesse den ben assaz,
que, com'aprendo, seu pan comendo,
foi mui correndo, parouss'en az.

Maravillosos ...

It's not portuguese, It's look like a creole dialect from portuguese mix with other languages.

We portugueses speaker dont undertand that The music says.

That is because the sound of standard Portuguese have more sonority (differences of sounds) and also do not use the gerund which brings advantages, since the use of the gerund limits the sounds, everything is equal. So a Portuguese person have more advantages in learn another language and also gor a perfect accent of the new language than a Brazilian. We also understand the Brazilians better than them understand us, because the 1st novels in Portugal was brazilians and at present still exist a lot of novels and channels Brazilians in Portugal. So they say we speak to fast :)

But that song is in Galician what is very easy to understand because is very close to portuguese.

The Portuguese language, also called Portuguese is a Romance language that originated in inflectional Galician-Portuguese spoken in the Kingdom of Galicia and northern Portugal.