@Svenmann According to this, https://freemarker.apache.org/docs/ref_builtins_string.html#ref_builtin_number, the number string has to be in computer language format, which apparently means it has to have a period as the decimal separators, and no thousands separators. Something like this might work, assuming all numbers have a comma for the decimal separator:

item.quantity?replace('.', '')?replace(',', '.')?number

thanks Scrott, it works !;)