a) the form and function of the katana is not really unique to Japan, the Gross Messer I mentioned earlier is very similar to the katana, and is European, but the name "katana" is more well known.
b) the scimitar may have been used in Europe, but is Middle Eastern in origin, and the name is derived from that. The name evokes Arabian nights, while the more Western name "saber" is more evocative of Europe.
c) Marco Polo visited China around 1300. In the 1400's European ships were travelling regularly to the Far East. Europeans and Japanese met at those times. Portuguese traders stopped at Japanese ports. The katana may not have been widespread, but there's no reason that one couldn't have shown up in Europe. Japan was largely an isolationist culture, but in an alternate universe, like in most fantasy campaigns, that same culture might be expansionist, and thus their gear, would spread.
d) The katana may be evocative of Japan and "oriental" adventures, but the same can be said of the scimitar, and you have no problem with them showing up.
And again, the katana isn't all that unique anyway (I regularly refer to it as a badly balanced, overly short, two handed saber), but is a much more well known name than messer, or yatagan (which, for the record, curve the other way).
My main beef is the katana isn't really special enough to deserve its own weapon entry. the Longsword, or scimitar entry covers it just fine.
