Pieces, pawns and chessmen In English as far as I know, pawns were once not pieces. A player had eight pieces and eight pawns which, together, totaled 16 chessmen.

When did this usage change? Why?

And (if you happen to know) how is it in the other major chess languages, like French, German and Russian?
The terms are a bit loose. To quote from Wikipedia:


  In chess, the word "piece" has three meanings, depending on the context.
  
  1) It may mean any of the physical pieces of the set, including the pawns. When used this way, "piece" is synonymous with "chessman" (Hooper &amp; Whyld 1992:307) or simply "man" (Hooper &amp; Whyld 1987:200).
  
  2) In play, the term is usually used to exclude pawns, referring only to a queen, rook, bishop, knight, or king. In this context, the pieces can be broken down into three groups: major pieces (queen and rook), minor pieces (bishop and knight), and the king (Brace 1977:220).
  
  3) In phrases such as "winning a piece", "losing a piece" or "sacrificing a piece", it refers only to a bishop or knight. The queen, rook, and pawn are specified by name in these cases, for example, "winning a queen", "losing a rook", or "sacrificing a pawn" (Just &amp; Burg 2003:5)


Source: Wikipedia, emphasis is mine.

From the same article, the term pieces to include pawns has been noted since the late 1980's. Speculating, it may be that commentators did not see the need to distinguish between the terms pieces and men, hence coming into common use. For example, Josh Waitzkin's tutorials in Chessmaster, first made in the 1990's, use the term pieces instead of men. I've seen terminology become more lax over time, e.g. pieces being 'killed' instead of 'captured'.

As for other languages, I am unable to say I'm afraid.