If we want to turn altcoin to cash, will we need to sell it for BTC first, and then send the BTC to service for conversion to USD/EURO before finally to bank?
It depends on the platform that you are going to use, because most of the time bitcoin, Ethereum, Bch, Xrp are the commonly use for encashment in terms of fiat currency. And as far as I know Bitcoin is the main useful in terms of encashment into a real money.