Wordpress - How to get a traslated string from a language other than the current one?

To find the answer to this question, you just need to look at how WordPress retrieves the translations. Ultimately it is the load_textdomain() function that does this. When we take a look at its source we find that it creates a MO object and loads the translations from a .mo file into it. Then it stores that object in a global variable called $l10n, which is an array keyed by textdomain.

To load a different locale for a particular domain, we just need to call load_textdomain() with the path to the .mo file for that locale:

$textdomain = 'your-textdomain';

// First, back up the default locale, so that we don't have to reload it.
global $l10n;

$backup = $l10n[ $textdomain ];

// Now load the .mo file for the locale that we want.
$locale  = 'en_US';
$mo_file = $textdomain . '-' . $locale . '.mo';

load_textdomain( $textdomain, $mo_file );

// Translate to our heart's content!
_e( 'Hello World!', $textdomain );

// When we are done, restore the translations for the default locale.
$l10n[ $textdomain ] = $backup;

To find out what logic WordPress uses to determine where to look for the .mo file for a plugin (like how to get the current locale), take a look at the source of load_plugin_textdomain().