Why is it, that in words like plaza / piazza, or blanca / bianca, the "l" in Spanish corresponds to an "i" in Italian? Is there a preference for this kind of sound in Italian, or is there another reason?
Spanish and Italian are both languages descended from Latin. As such, many of their words are cognate sharing a common Latin ancestor, but the sounds in these words evolved over time and evolved differently in each language.
In Spanish, pl-, fl- and cl- generally became ll- (pronounced the same as Italian 'gl'):
As regards /pl-/, /ɸl-/ and /kl-/, these have /ʎ/ as their normal reflex in Old Spanish and Modern Spanish:
- plānum > llano ‘flat’
- flammam > llama ‘flame’
- clāvem > llave1 ‘key’
Note, however, popular exceptions such as
- plateam > plaza ‘square’
- flōrem > flor ‘flower’
- clavīc(u)lam > clavija ‘peg’.
(The sequences [pl], [ɸl] and [kl] also occurred in postconsonantal position, and in that case the reflex is the affricate /tʃ/, generally with loss of the preceding consonant if this is not a nasal:
- amplum > ancho ‘wide’
- inflāre > hinchar1 ‘to swell’
- masc(u)lum > macho ‘male’ )
In Italian, the 'l' in these clusters became an 'i':
- planum > piano
- flammam > fiamma
- clavem > chiave
- plateam > piazza1
- florem > fiore
- clavīc(u)lam > cavicchia1
- amplum > ampio
- inflāre > enfiare
- masc(u)lum > maschio1
1. Though Italian does have the occasional doublet of these words retaining the Latin consonants, these are invariably learned borrowings or loanwords via other Romance languages that didn't undergo these changes, e.g. platea (borrowed); caviglia (through Old Provençal), clavicola (borrowed), masculo (borrowed) etc. Such learned borrowings also occur in Spanish e.g. clave, inflar etc.
I would say, /l/ in specific Latin clusters was simply vocalized in Italian. That means the consonant became a vowel, which is not all that uncommon for a sound like this. Take for example r-vocalizations in German wer /veːɐ̯/ and English bear /bɛːə̯/. /l/ has been vocalized in Cockney, I think, feel [fiu̯], in Dutch goud 'gold' and Bavarian German varieties spui german 'Spiel'.
In an l-sound some part of the tongue touches the roof of the mouth but [!] lets the air escape at the sides[!]. If a speaker does not do this, what remains is a vowel because the air stream is unhindered. This can sometimes facilitate the articulation.
Just for (a random) curiosity :D : Latin pl, cl and fl bacame [ʃ] in portuguese (written ch).
Example: planum > chão (doublet of "plano") plattus > chato (doublet of "prato") plenum > cheio (doublet of "pleno") clamare > chamar (doublet of "clamar") clave > chave ("doublet of "clave") flama > chama (doublet of "flama") flor > chor (later replaced by the relatinazed word "flor")
Except in words that entered later in the language, in this case, pl, cl, fl etc became pr, cr, fr etc.
Exemple: plata > prata plato > prato claro > craro (later replaced by the relatinazed word "claro") flecha > frecha (later replaced by the relatinazed word "flecha")