To understand this term, you have to understand a minimum about the US voting system and in particular, the electoral college. To win the election, a candidate must win 270 of the 538 electoral votes in the country. If you win a plurality of votes in a state, you win *all* the electoral votes. If 49.01% of Florida's voters vote for Kerry and 48.99% vote for Bush, and 2% vote for Nader, then Kerry gets *all* Florida's 27 electoral votes. (This is exactly why there was such a furor in 2000, when a switch in 269 voters' ballots in Florida from Republican to Democrat would have switched *all* of Florida's (then) 25 electoral votes from Bush to Gore, thus electing Gore President in 2000.)
So, since a tiny change in the numbe of votes in a state can cause *all* of the state's electoral votes to switch from Democrats to Republicans or vice versa, a 'swing state' is a state which is unpredictable and could swing either way. If the election is close nationwide, then a swing state could swing the whole nation, as Florida did in 2000.
This could be due to undecided voters, but not necessarily. It could also be due to a very close race which cannot be predicted, even though all voters made up their mind long ago.
This is probably why this term is not translated--there's too much involved. Undecided is really not a good translation, imho. Rather, if translated at all, it should involve some term like a fulcrum, pivot, balance point, tipping point, seesaw, or something like that.