I mean I want to log income, spendings, exchanges in multiple currencies (or commodities), and then be able to see reports in a single currency - my net worth, my income over period of time, capital gains / losses.

Leger printed nonsence for such roports.

I used to pay taxes in multi currency environment, and know the official rules here, in Belarus, how to calculate the indicators when converting to a single currency.

Also I once came across US guideline for that - the same simple principle.

So I guess that's a very standard aporoach in accounting.

Ledger does something strange instead.

I remember also some large blog post was circulating, where the author "explains" how to do accounting in two currencies, in some overcomplicated way, that only works for two currencies. This link was usualy shared between people as an ansver to the multi-currency question.