refresh translations (no manual change)
Release 2.4.9

  Recursion lesson is usable again, sorry for the inconvenience.
  (we really need more tests, grmbl)

  * Engine:
    - Lesson constructor waits for all init threads' completion
  * Worlds:
    - Turtle: equal() was too permissive after recent work on merging
      lines that were too picky. Now take the world name into account
      in equality, so that we can switch from an empty world to
      another empty world in the UI.
  * Pedagogical content:
    - recursion: remove the largest fractals that were too heavy
    - #366: recursion.spiralUse: add another nice picture