In the mode you’ve probably been already, the one you set the starting points from, right-clicking on each contour should give you a contextual menu with some very basic contour-ordering commands. But it won’t be very effective either. I haven’t done much FontLab scripting in particular, but looking at your contours, just finding the lowest y coordinate in each contour and sorting the contours by this value should do, algorithmically.

Is there a reason why do you want them sorted? Because the only reason I myself can think of is the file size, as the bigger the distances between points, the bigger the numbers that need to be stored. In case of a TrueType binary, for example, the threshold is at 256 font units, above which storage requirements for a particular distance double, but that’s still mere two bytes. Machine movement of some kind?

Thank you, Grzegorz,
The right-click menu helps :)

It involves some clicking, but it's working fine.
I'm not experienced in scripting at all, luckily the number of glyphs in my font is not that high.

The whole sorting "problem" is probably not that big deal - I was rather concerned about potential interpolating issues.

Thanks,
(v)

removed

In Glyphs > CMD+Shift+R

By the way, it looks awesome!