The account currency is the currency of your balance in your trading account.
In any corner of the world, you can store funds and make deposits or withdrawals in any currency you choose. You will find it easier to control your trading account balance if your funds are stored in the same currency as most of your deposits and withdrawals.
The first consideration when choosing a particular currency is the convenience of making deposits and withdrawals.
For example, if you usually replenish your account in USD, then it would make sense for you to choose USD as your trading account currency. If your deposit currency is different from your account currency, the amount of your transaction will be converted at the current exchange rate.
Any change in the exchange rate of your account currency will affect the amount of your deposit. Pay attention to the stability of your chosen currency.
Suppose you choose euros (EUR) as your account currency but make a deposit in US dollars (USD). After making a 1300 USD deposit, you have 1000 EUR deposit (given a 1.30000 EURUSD exchange rate). Later, after completing several profitable transactions, you have increased your account balance by 20%, i.e. 1200 EUR. Then you decide to make a withdrawal. However, in the meantime the EURUSD exchange rate has changed to 1.20000. After converting the 1200 EUR, you could withdraw 1200 x 1.20000 = 1440 USD. You deposited 1300 USD and withdrew 1440 USD. Thus, your actual profit after the conversion is a little more than 10%, rather than the 20% received from trading.