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What makes an expense tracker actually multi-currency

Almost every budgeting app says it supports multiple currencies. Very few mean the same thing by it, and the difference only shows up a month after you have moved, when the numbers stop making sense. Here is what the phrase can mean, and the five questions that tell the versions apart before you commit your history to one of them.

Three different things called "multi-currency"

The label covers three levels of support, and the gap between them is large.

Display conversion. The app has one currency. You can ask it to show totals in another, converted at today's rate. Everything you enter is stored in the single base currency.
Per-transaction currency. You can tag a transaction as being in another currency. The app converts it to the base currency on the spot and stores that.
Per-wallet currency. Each account holds its own currency for real. A euro account is in euros, a dinar account is in dinars, and nothing is converted on the way in. Totals are converted; the records are not.

Only the third one survives living in two currencies. In the first two, a rate change rewrites your history: the coffee you bought for 300 dinars last March becomes a different number of euros every time rates move, and your February and March stop being comparable.

The five questions

Ask these of any app before you type a year of history into it.

Can two accounts hold different currencies at the same time? If the app asks for one "main currency" at setup and never offers another, it is doing display conversion.
When I record 300 in a foreign currency, what gets stored — the 300, or its converted value? The 300 is the fact. The converted value is an opinion that changes daily.
What happens to a transfer between two currencies? This is the sharpest test, and the one most apps fail. See why the rate on a transfer matters.
Does it need my bank to be connected? Outside the US, UK and EU, most banks connect to nothing. If automatic import is the app's whole premise, it has no premise where you live.
Can I see one total across everything? Per-wallet currencies are useless if the app cannot add them up into the one number you think in.

How Walletable answers them

Every wallet has its own currency, chosen when the wallet is created, and the amounts inside it are stored as you entered them. The total at the top of the Wallets tab is converted into your main currency at current rates, and that conversion is the only place rates are used — your records are never rewritten by a rate change.

A transfer between two currencies keeps both amounts: what left one wallet and what arrived in the other. There is no bank connection at all; where banks send text messages, expenses are created from those on the device. There is no limit on wallets, currencies or transactions, and that part is free.

Questions

Is a multi-currency tracker the same as a currency converter?

No. A converter answers "what is 300 dinars in euros right now". A multi-currency tracker keeps a ledger in several currencies at once and converts only the totals, so last year's spending still reads the way it did last year.

Do I need one if I just travel occasionally?

Probably not. A trip is a closed episode: you change money, spend it, come home. The case for a real multi-currency ledger starts when you are paid in one currency and live in another month after month.

Does the main currency have to be the one I earn in?

It should be the one you think in. For most people who have relocated that is the currency of the rent, not the salary — the number you want is "how much do I have here", not "what is this worth back home". It can be changed later without converting anything.

How many currencies can one app realistically handle?

In Walletable there is no limit, and the practical answer is three or four: what you hold locally, what you are paid in, and what you save in. More than that is usually a sign the wallets want merging, not that the app is short of room.