Amounts quoted in sterling, US dollars and euros do not normally need to be converted. The only exception might be if you want readers to be able to make direct comparisons between countries: if that applies you should choose one as the standard.
Any other currency should be converted the first time it is mentioned to the equivalent in US dollars. There are a number of currency converters on the net: xe.com covers any currency you are likely to encounter.
Currencies change from day to day, so exact conversions are inappropriate. Round them up or down to a sensible figure. Remember too that official exchange rates can be misleading when it comes to people’s purchasing power, and alternative conversion rates may be more appropriate, where available.
Take care too with historical comparisons: exchange rates can shift over the long term.