I think the Gaelic referred to in the site I looked up is probably not modern Irish. Here the names they mention :
20 fiche
30 deich fichead (10+20)
40 daichead (2x20)
50 deich is daichead (10+2x20)
60 tri fichid (3x20)
ETA : I notice that all the examples I needed were there, in the various relevant languages. I had no need to make some up.
Danish is decimal up to 40.
2 = to, 20 = tyve
3 = tre, 30 = tredive
4 = fire, 40 = fyrre
From 50 it is vigesimal, but with some added weirdness. The word “Halv” (Half) followed by an ordinal number means the corresponding cardinal number minus one half. So “Halvanden” (half second) = 1½, “Halvtredie” (half third) = 2½, “Halvfjerde” (half fourth) = 3½.
The old word for fifty was “Halvtredsindstyve” (half third times twenty). This has contracted to “halvtreds”.
60 = tres ( 3 x 20)
70 = halvfjerds ( ½ 4. x 20 )
80 = firs ( 4 x 20 )
90 = halvfems ( ½ 5. x 20 )
“Halvanden” is still in ordinary use, “halvtredie” etc. is only ever used for explaining why the numbers are called what they’re called :-).
Deich fichid is odd, because it looks like it would mean “10 20s = 200”, not “20+10 = 30”. More likely would be “fiche is a deich”. I don’t think it’s used anymore, but you can imagine it being used 100 years ago or in Scottish Gaelic.
Old Irish (the ancestor of Modern Irish, Scottish Gaelic, and Manx) has a complete set of decimal numbers for 10, 20, 30, 40, 50, 60, 70, 80, and 90. All the Celtic languages use base-20 to a greater or lesser degree, but the underlying Indo-European decimal is always there, too.
ETA: I agree about “deich fichead”: surely that should be “deigh a(gu)s fichead” or something.
The oddest counting system I have ever encountered was Danish where, for example (pardon my errors; this is from dim memory, but the idea is right) 55 is rendered “fem og halv tres”, literally “five and half third”, meaning five + half-way through the third [twenty]. I believe they are attempting to reform it so that younger people will say femti fem, with the obvious meaning.
Not exactly.
50 = “halv treds” = half [before] three (=2,5) times twenty
60 = “treds” = 3 times twenty
70 = “halv fjerds” = half [before] four (=3,5) times twenty
80 = “fjerds” = four times twenty
90 = “halv fems” = half [before] five (=4,5) times twenty
“treds”, “fjerds” and “fems” are derived from “tre, fire and fem sinds tyve” (“sinds” is an old word meaning (mathematical) “times”, and I have no idea if it is used in any other context).
**ETA **
Don’t count on any success. Norway had a reform over fifty years ago when they changed it from “fem og femti” to “femtifem” to no or little avail.