From the document.getElementsByTagName I guess you are running the javascript in a browser.
The traditional way to expose functionality to javascript running in the browser is calling a remote URL using AJAX. The X in AJAX is for XML, but nowadays everybody uses JSON instead of XML.
For example, using jQuery you can do something like:
$.getJSON('http://example.com/your/webservice?param1=x¶m2=y',
function(data, textStatus, jqXHR) {
alert(data);
}
)
You will need to implement a python webservice on the server side. For simple webservices I like to use Flask.
A typical implementation looks like:
@app.route("/your/webservice")
def my_webservice():
return jsonify(result=some_function(**request.args))
You can run IronPython (kind of Python.Net) in the browser with silverlight, but I don't know if NLTK is available for IronPython.