> Does anyone know of a list (or can we compile one between us) of languages which use the 2nd person plural as a polite form for the singular.
Almost all the Indian languages will go into the list. Bengali (or Bangla) has only 2nd person plural and no familiar form for the Singular. Hindi/Urdu has three levels :
- the 2nd person plural as a polite form (aap)
- a part polite- part familiar form, distinct from the 2nd person plural (Tum)
- a familiar form (Tu)

This reminds me of lines from a Ghazal by the Pakistani singer Mehdi Hassan. The translation :

As love passed all limits, all ceremony we overcame
First 'aap' then 'tum', and then worthy of 'Tu' became.


For those who know the language this is the original:

Pyaar jab hud se badhha, saare takalluf mit gaye
'Aap' se phir 'tum' huye, phir 'tu' ka unwan ho gaye.