Should you invest in foreign currencies? I'm not against investing in foreign currencies for speculation. At the same time, there are better plays to hedge against the U.S. dollar.
If you live in the U.S. and work in the U.S. and your income is in U.S. dollars, then it is important to diversify yourself out of U.S. dollars. But remember that your debts are also in U.S. dollars. If you have a fixed rate mortgage, then you don't really have to worry about inflation as far as that goes. You do have to worry about your investments and your general cost of living.
While investing in foreign currencies may provide a hedge against a falling dollar, it is still a speculation. All of the major currencies used on this planet are fiat currencies. They are not backed by gold, silver, or anything else other than the taxing and printing power of the governments. You could invest in Japanese yen or the euro to diversify out of the U.S. dollar. It might work, but they might also go into free fall together.
That is why gold is still the best investment to hedge against the U.S. dollar. You don't have to worry about someone printing gold. The only way there will be additional gold is by the hard work of mining. Even then it is unlikely that there would be an explosion in the supply of gold.
If you want to speculate in the euro, yen, yuan, New Zealand dollar, or any other currency, then go for it. Just remember that, just like the U.S. dollar, it is a fiat currency.