I have learned from Avva that Swedish, which used to have a formal/informal pronoun distinction Ni/du comparable to French vous/tu or German Sie/du, has virtually lost it, and the change occurred in a remarkably short time. The origins of the change are recounted in a fascinating discussion at soc.culture.nordic; as Jan Böhme explains,
Bror Rexed, when he became General Director [of] the National Board of Social Issues and Medicine (Socialstyrelsen), in 1968(?), issued a formal decree[...] that he wanted to be called by first name and “du”, and expected the rest of the staff to do the same.
The development was considerably speeded up when Olof Palme, as new Prime Minister in 1969, let reporters call him “du” on live broadcasts.
One reason the change occurred so quickly is that Swedes traditionally addressed anyone with a title by that title, using the third person: “Would the professor like more tea?” (Jan Böhme’s father was called “Mr. Appeals Court Justice” until the late ’60s.) Thus the use of “Ni” was slightly derogatory, implying that one’s interlocutor had no title or office worth bothering about. With that kind of system, it must have been a relief to start using one pronoun for everybody. The interesting thing is that, according to Jon Kåre in the same discussion,
Norway almost immediately followed Sweden in adopting “du”, although our polite form of address was simply “De”. That is, the system was like in French or German, and not at all like in Swedish.
If anyone knows anything more about this, please leave a comment.
Avva speculates on the possibility that Russian itself might lose its parallel Vy/ty distinction within a generation, since young people routinely use “ty” with each other, but decides it’s unlikely because of the ingrained use of the distinction to reinforce subordination in the workplace: the boss addresses his underlings as “ty” and they must respond with “Vy.” Avva despises this (as would I), but since he lives in Israel he doesn’t have to put up with it.
Addendum. Avva points me to a discussion by Dmitri Evmenov of the origin and history of Swedish Ni; it was originally I, borrowed from German, and became Ni through reanalysis (ären I > äre ni).
Further addendum. Des says Ni is making a comeback (thanks to SAS)! See comment #4 within.