I have a Drupal 7 webpage already developed with all texts in Spanish, but now I want to add English language to the page.
As all my texts are written in spanish, I would like to know what is the best/fast way to translate it to english. I tried with t() function, something like this t('Hola mundo') expecting to be manually translated by me with Drupal translation module (admin/config/regional/translate/translate) to 'Hello world'.

The Drupal 7 documentation says this:
$string A string containing the English string to translate.

So my question is: is there any way to translate with t() from the default language (spanish)?

If it is not possible, how can I translate all the texts to english from spanish?

Thanks