How I'd do this

comment.js

var field = document.getElementById('id_needs_change_comment');
var maxlength = Number(field.getAttribute('data-maxlength'));
var remaining = document.getElementById('comment-remaining-characters');
var remainingText = remaining.textContent;

field.addEventListener('keyup', function() {
    var remainingChars = maxlength - this.value.length;
    remaining.textContent = remainingText.replace('%d', remainingChars);
}, true);

forms.py

    needs_change_comment = forms.CharField(
        # ...
        widget=forms.Textarea(attrs={'data-maxlength': 500}),  # Preferably, don't hard code 500 but get it from the model.
        # ...

template.html

<span id="comment-remaining-characters">{{ _('%d characters remaining') }}</span>