In terms of how it gets redirected, try make sense from this: https://qtranslatexteam.wordpress.com/browser-redirection-based-on-language/. Your browser probably already had cookie for English, or has English as preferred language. I am pretty sure you will figure that part out.
Second question, just use, “<!–:es–>http://www.palorosa.mx/es/contacto<!–:–><!–:en–>http://www.palorosa.mx/en/contacto<!–:–>” instead. The explanation why is in the same document already cited above.
As a side note, use of “<–:–>” works, but it is obsolete: https://qtranslatexteam.wordpress.com/multilingual-fields/.