localeParseFloat.js

localeParseFloat(s, locale) {
    // Get the thousands and decimal separator characters used in the locale.
    let [,thousandsSeparator,,,,decimalSeparator] = 1111.1.toLocaleString(locale);
    // Remove thousand separators, and put a point where the decimal separator occurs
    s = Array.from(s, c => c === thousandsSeparator ? "" 
                         : c === decimalSeparator   ? "." : c).join("");
    // Now it can be parsed
    return parseFloat(s);
}

console.log(parseFloat(localeParseFloat("1.100,9"))); // user's locale
console.log(parseFloat(localeParseFloat("1.100,9", "us"))); // US locale
console.log(parseFloat(localeParseFloat("1.100,9", "nl"))); // Dutch locale: reversed meaning of separators
// And the same but with separators switched:
console.log(parseFloat(localeParseFloat("1,100.9"))); // user's locale
console.log(parseFloat(localeParseFloat("1,100.9", "us"))); // US locale
console.log(parseFloat(localeParseFloat("1,100.9", "nl"))); // Dutch locale: reversed meaning of separators