var x = parseFloat(document.getElementById("Lang_from").value);

var x = parseFloat(Lang_from.value);

var x = parseFloat(Lang_from.value);

var x = Number(Lang_from.value);

var x = + Lang_from.value;

const x = +Lang_from.value;

const lf = +Lang_from.value;
const lt = +Lang_to.value;
const q =  +quantity.value;
const s =  +subject.value;
const ft = +file_type.value;

var total = (+Lang_from.value) + (+Lang_to.value); // The extra + to coerce to number
total *= quantity.value;  // *= same as total = total * quantity.value;
                          // The extra + not ended if the operation on the value is
                          // not +
total += +subject.value;
total += +file_type.value;

addEventListener("load", function() {  // waits for the page to load
     calculate.addEventListener("click", calculateTotal);
});

// or using arrow function and a slightly earlier event that fires when
// all the HTML content has loaded and been parsed (image and the like may not be ready)
addEventListener("DOMContentLoaded", () => {  // waits for the page to load
     calculate.addEventListener("click", calculateTotal);
});


// NOTE that the element id is calculate which conflicted with the JavaScript function
// name calculate. I renamed the function to calculateTotal ( which I would have done
// even if there was no name conflict)