what is the world standard??
To answer the original question, I believe that knots, mile per hour or km per hour are all acceptable units of speed. Feet seem almost universal for height except in eastern europe.
Why actually do we use knots? The definition of nautical mile (one minute of arc on a great circle) is fairly irrelevant when flying from Norwich to Perth (for example). We could as easily use statute miles and mph (or km if we want to be good Europeans). From the point of view of the POH the unit is fairly irrelevant - you just fly the numbers, which I believe in Yaks is in km/hour.