What are your opinions on language changes within lyrics? Like, nothing crazy like English to Mandarin or Afrikaans but from English to French/Spanish?

Personally I think it sounds cool and has a cool effect if done right.

Examples of this are mostly from Arcade Fire and the Pixies

Haiti - Arcade Fire
Haïti, mon pays,
wounded mother I'll never see.
Ma famille set me free.
Throw my ashes into the sea.

Mes cousins jamais nés
hantent les nuits de Duvalier.
Rien n'arrete nos esprits.
Guns can't kill what soldiers can't see.

In the forest we lie hiding,
unmarked graves where flowers grow.
Hear the soldiers angry yelling,
in the river we will go.

Tous les morts-nés forment une armée,
soon we will reclaim the earth.
All the tears and all the bodies
bring about our second birth.

Haïti, never free,
n'aie pas peur de sonner l'alarme.
Tes enfants sont partis,
In those days their blood was still warm

Crackity Jones - The Pixies
Jose jones
Told me alone
His story
He got friends
Like paco picopiedra
La muneca
He receives on his set
Crack crack crackity jones

Please forgive me
Jose jones
You need these walls
For your own
I'm moving out of this hospedaje
I'm afraid you'll cut me boy

Thirty miles by
Hundred miles by
Stinking island
Por goofiar
En crushing automovil
Chasing voices
He receives in his head
Crack crack crackity jones
I love when it throws the foreign words in.. like in Five Finger Death Punch's song Under and Over It's 2nd verse finishes with "C'est la vie!.. Adiós!.. Good Riddance!.. **** YOU" and it's really good to shout along to xD... also, sum 41 did their song "Ma Poubelle" fully in french, which has an amazing touch.. i love songs in french/spanish;o or italian is good;p
It can work quite well

Orphaned Land combines several languages with English, I think mostly Hebrew and Arabic and it's pretty cool.
It's called Macaronic language. I would avoid it if you're intentions are to write something serious. If you do want to use it in a serious work, then use a word or phrase that doesn't exist in English, or where the appropriate translation is long-winded.
I think my personal favorite example is the French hook from "Lady Marmalade."