Why the dashboard has a second currency
The invoice is in the currency the client wires. Your dashboard totals in USD, and shows your local currency underneath.
One PDF, one currency
The client should not have to guess which figure to pay. The PDF carries the wire amount only.
Your books still need a home currency
Set the company location in settings. Dashboard and reports stay in USD, with a second line in the local currency for that place. That line is for you. It is not printed on the invoice.
The rate is not a price you quote
For a company in mainland China, the local line uses the rate you save on that invoice. Elsewhere it uses the day's rate. It is a book figure. Do not send it to the client as the amount due.