Thought it should have its own post, eventhough coming up as a subpost in Building a hydrofoil
Is my suggestion.
Mast or Keel is what I've heard most, also manufacturers, use.
Mast is a bit confusing, i admit...
Keel is technically okay, as it will act as a keel fully or a small part of it, always, depending on height in the water.
In dinghys, retractable keels does NOT go lengthwise, but can be deeper than the chord, underwater.
Also on some yachts, sometimes called a "Fin Keel" to emphasize it is not a lengthwise type.
I dont like the name "riser", and "strut" is also impossible to translate to most other langauges.
Some not into kitesurfing, always look at you like you are from another planet, when you say "struts" about the kites struts.
"Battens" (or even sail battens) is what they understand immediately (also in other languages)
"Tower" could be an appropriate alternative, and can easily be translated to other languages
Just some ideas