How to create a file and append data from html page ?
Browsers have lots of security that prevent this level of control over your computer.  This is a good thing. You dont want random websites to be able to do this stuff on anyone's computer that visits them.
They way to do this would be to write a web application that your browser could access. The browser can submit data to this application running on your own computer and your application could manipulate the file system or do lots of other things.
So no, a browser can't do these things. And yes, you would have to use "another language" to create something which runs outside the browser itself. You can use javascript (see node.js) or python to do this, as well nearly any other programming language that exists to create such a thing. Which to choose is up to you.
I have an html file on my desktop that takes some input. How would I go about writing that input into a file onto my computer? Would I have to use another language to do it (i.e python or javascript?) and how would I go about doing this? On a related note, is there any way I can have javascript start an application from within an html file (the goal is to write to a file on my computer?