An amusing guide in the style of a flowchart on how to navigate the moral/linguistic maze that is the familiar/formal address systems found in many languages, known to linguists as the "T-V binary" (refers to tuvous in French).
The TuVous maze
Many languages have a familiar/formal address differentiation which has been lost from English. This differentiation can be used to distance oneself from someone e.g. a stranger or a subordinate or, conversely, to endear oneself say, to a child or good friend. Some languages, like French, have a strong culture of maintaining the formal divide until both parties agree to move to the familiar. Other languages, like Spanish and Belgian French, dispense with the formal very quickly. It is probably better to talk of "culture" here rather than "language", as it is the speakers' culture which decides when to use the familiar/formal address system rather than the rules of the particular language.
The fact that linguists use the French words tu and vous in the academic description of the phenomenon is indicative of a) the enduring popularity of French as a language of study and b) the historical importance of the French language. In my view, it is also an indication of the respect afforded by academics and language students alike to the high status given by French culture and society to the French language and its highly formalised grammar. From a translator's point of view, there is a lot to be said for French as a source language given that the written language is very standardised; from official documents, through to journalism down to everyday writing, everyone adheres to the long-standing rules of French grammar. This is in contrast with Italian, where the rules of grammar are no less formally codified, but unfortunately writers of the language do not feel the need to stick rigidly to such rules. This can lead to much frustration on the part of the translator who is trying to decypher the intention of the writer.
Here's a more detailed description of the mechanism in French explained.
Comparison of mechanism in various languages I = informal, F = formal:
French: Tu (I) Vous (F) (2nd person plural)
Spanish: Tu (I) Usted/es (F) (3rd person singular/plural)
Italian: Tu (I) Lei (F) (3rd person singular)
Polish: Ty (I) Pan/Pani/Państwo (F) (3rd person singular)
As you can see, more languages use the 3rd person to denote formal address, even though the speaker is actually addressing someone in the 2nd person.