>     // remove accent

Actually, no. The following code is just a lossless conversion to and from UTF-8 which doesn’t change the text.

In the following, I’d coalesce the regular expressions – if nothing else, this is way more efficient. I’d also import the namespace to get rid of this overlong explicit namespace qualification. The “collapse spaces” phase makes no sense since you’ve already *removed* spaces.

Finally, you can also coalesce the `Trim` statements.

Ignoring for now that the accent removal doesn’t work, this leaves us with:

    input = input.ToLower();
    
    // remove stop words, entities and anything that is not letters, numbers, dash, or space
    string stopWords = string.Format("\\b{0}\\b", string.Join("\\b|\\b", ENGLISH_STOP_WORDS));
    input = Regex.Replace(input, stopWords + @"|&\w+;|[^a-z0-9\-\s]", "");
    
    // replace spaces
    input = input.Replace(' ', '-');
    
    // collapse dashes
    input = Regex.Replace(input, @"-{2,}", "-");
    
    // Trim dashes and spaces
    input = input.Trim(' ', '-');

Finally, to remove accents, you need to [*normalize* the Unicode string so that accented characters are decomposed into diacritics and remove combining diacritic marks](http://blogs.msdn.com/b/michkap/archive/2005/02/19/376617.aspx):

    static string RemoveDiacritics(string stIn) {
        string stFormD = stIn.Normalize(NormalizationForm.FormD);
        StringBuilder sb = new StringBuilder();
        
        for(int ich = 0; ich < stFormD.Length; ich++) {
            UnicodeCategory uc = CharUnicodeInfo.GetUnicodeCategory(stFormD[ich]);
            if(uc != UnicodeCategory.NonSpacingMark) {
                sb.Append(stFormD[ich]);
            }
        }

        return sb.ToString();
    }