When it comes to currencies, the term "speculation" is more appropriate than "investment". "Investment" implies that you are providing financial resources for some economic activity, and receiving a portion of the resulting benefit. Investment is not a zero-sum game: if a venture capitalist invests money in a start-up that provides a service that increases productivity, then there's a net increase in wealth across society, and the venture capitalist will receive a portion of that. Speculation, on the other hand, is zero-sum: any profit you make isn't from net increase in societal wealth, it's just money you're getting from other people. Investing is where you get money by having your money provide a service to the economy, while speculation is where you try to get money by being "smarter" than other people.
If you just buy something and don't have that thing participate in the economy in any way, that's not really "investing". So if you have a vault filled with yen, pounds, gold, silver, paintings, baseball cards, bright shiny bitcoins, etc., that's not "investing". (A vault full of grain could be considered investing if there are seasonal variations in the price of grain and you're providing the service of storing grain from harvest until winter).
If you are going to store your wealth in currency, there is benefit in diversifying into several different currencies. But as Glen Pierce says, currencies have a general downward trend known as "inflation". If you want your money to keep its value, you need to find some way of having it provide economic value, such as stocks or bonds.