What I needed was the inline notation for f:format.currency. Usually it looks like this:
<f:format.currency currencySign="€" decimalSeparator="," thousandsSeparator="." prependCurrency="false" decimals="2">{somevar}</f:format.currency>I needed this in an attribute and therefore needed the inline notation. The solution:
{somevar -> f:format.currency(currencySign:'€',decimalSeparator:',',thousandsSeparator:'.',prependCurrency:'false', decimals:2)}