Base currency
Set in Settings → Currency → Base currency. New values are entered in it, totals are shown in it, and price feeds are asked for it.
Items in other currencies
Any value can be in another currency — a watch bought in euros, a flat valued in pounds. Each keeps its own currency; nothing is ever rewritten.
To count them in totals, record an exchange rate in Markets → Exchange rates → Add rate: the From currency, To currency, Rate and As of date. Either direction works.
Totals convert at the latest rate on or before each total's date, and say which rates were used. Without a rate, those items are listed but left out of totals, with a note saying so — never converted at a guess.
Earlier rates are kept (Earlier rates) so past totals stay right. Delete a mistaken rate with its trash button.