Strings

  key: Some String
  key: 'Some %1$s String to %2$s'
  ...

see also SimpleFormatting_

Numbers

  key: 123
  key: 123.45
  ...

Pluralizable messages

  key: !!pl
    0: No fools
    1: One fool
    n: %d fools

Above was an example for English. Please see Pluralization for other languages.

Procedures

  key: !!proc |x, y| x + y