Whether you use an altcoin (or bitcoin) for a long or short-term investment, or because it satisfies you as a medium of exchange, it'd be misleading to call it a "currency".
If you use a fork to scratch your back, that doesn't mean a fork is no longer a kitchen utensil.
A currency is a system of money in general use in a particular country.
Wrong, that is the definition of fiat issued by government not the meaning of a currency. And you are using the dictionary definition here.
A currency is simply any medium of exchange. For example in a game the gold coins your character earns is a currency since you can use them as a medium of exchange in that world to buy goods such as an armor!
altcoins (including Bitcoin)
Your terminology is all over the place!
Altcoins or more precisely alternative cryptocurrencies are cryptocurrencies other than bitcoin. They don't "include" bitcoin.
Saying that Bitcoin is a currency means that, hypothetically, it could replace a national currency if the nation said so.
No it does not mean that.
Saying bitcoin is a currency means you can use bitcoin as a medium of exchange