Selecting aggregated string from database

I have a table in my database consisting of one column which is NewLanguage(varchar). I insert new languages to the table and I want to select the rows of the table and assign them to a string variable delimited by ‘,’. I try to select the languages as in the code:

    public string  SelectLanguagesFromDatabase()
    {
        try
        {
           NpgsqlCommand command = new NpgsqlCommand(
             "SELECT string_agg("NewLanguage",',') FROM "Languages")", 
             dbConnection);

           results = (string) command.ExecuteScalar();
           command.Prepare();
        }
         catch( Exception ex){
             MessageBox.Show(ex.ToString() );
         }
        return results;
    }

When I test it, results returns as null. I want to take string representation of the rows’s values separated by ‘,’. How can I do that? Thanks.