Multi-currency finances used to be rare. Today, if you are an expat, a remote worker, a freelancer with international clients, or simply someone with savings in two countries, you are running a multi-currency household whether you planned to or not.
The first question to answer is: what is your reporting currency? This is the currency you want your total net worth expressed in. It is usually the currency you live in, but not always. Pick one and stick with it — switching back and forth makes your trend lines noisy and confuses every comparison.
Each account stays in its native currency. Do not convert balances by hand and store the converted value — you lose the source of truth and you bake the exchange rate of one specific day into your history. Store balances natively, convert on display.
Exchange rate volatility is real but rarely catastrophic over a year or two. A 5-10% FX move on a balanced portfolio is normal noise. If a single currency makes up a very large share of your wealth, that is a concentration risk worth being aware of, but it is not a reason to panic-rebalance every quarter.
A practical heuristic: hold expenses in the currency you spend in, hold long-term investments in a globally diversified portfolio (which already gives you implicit currency diversification), and do not try to time FX moves. The simple version is almost always good enough.