function to change language object
The list of language objects is built up by the core function language_list(), which you can use to overwrite the global $language object if you need to:

// Get the list of languages
$languages = language_list();

// Set up the new language code
$new_lang_code = 'fr';

// Make sure the required language object is actually set
if (isset($languages[$new_lang_code])) {
  // Overwrite the global language object
  global $language;
  $language = $languages[$new_lang_code];
}