German formal 2nd person singular 'Sie' is indeed identical to 'ordinary' 3rd person (plural?).

Usage nuances of 2nd person singular vs. plural differ quite a lot among languages (not only Romance - Croatian has similar pattern). For example, in Croatian formal plural 'vi' (as opposed to singular 'ti') will be simply used as a sign of respect (so, two stangers arguing - e.g. drivers - will often not use it). In some other languages it will be used also to denote distance (and singular will be used only when addressing a small child, lover or closest of friends). Fascinating.

Concerning 'thou' and 'you', I should check my sources (and memory) better.