foo.js

function reverseFormatNumber(val,locale){
        var parts = new Intl.NumberFormat(locale).formatToParts(1111.1);
        var group = parts.find(part => part.type === 'group').value;
        var decimal = parts.find(part => part.type === 'decimal').value;
        var reversedVal = val.replace(new RegExp('\\' + group, 'g'), '');
        reversedVal = reversedVal.replace(new RegExp('\\' + decimal, 'g'), '.');
        return Number.isNaN(reversedVal)?0:+reversedVal;
    }

console.log(reverseFormatNumber('1,234.56','en'));
console.log(reverseFormatNumber('1.234,56','de'));

I think Number.isNaN(reversedVal)?0:+reversedVal always returns +reversedVal because calling Number.isNaN on a string is always false. Number.isNaN(+reversedVal)?0:+reversedVal (note + inside the isNaN call) could be used to return 0 if the string does not appear to be a number.