Sorcha,

I don't think English ever had a second person plural informal. Spanish does:

	Singular	Plural
First Person (I/we)	yo	nosotros
Second Person (you/y'all) Informal	tú	vosotros
Second Person Formal	Usted	Ustedes
Third Person ([s]he/they)	el ella	ellos ellas

Admittedly, while the second person singular informal is used quite a bit -- in particular, en famille (to mix languages *G*) -- the second person plural informal is almost never used today. It's my understanding that it's only taught in schools so kids can understand writing of three or four generations ago.

BB,
NightWing