I spell install with two L's here in Canada, and I believe it is an English way of spelling it, so probably the same throughout the Commonwealth. The one L might be a mistake or perhaps either way was / is used.
From the Merriam-Webster dictionary, there are two spellings, with and without the second L. The original meaning seems to have come from
Quote:
1 a : to place in an office or dignity by seating in a stall or official seat
Webster, I think, worked on shortening the English words to an easier to use spelling.
I suppose it depends on where you learned your letters, which one you consider to be correct.