Helper global functions to interact with Polylang plugin

Extends Polylang include/api.php with new methods.

• pll_get_languages_list Gets an array with the cached infomrmation about installed languages.

• pll_get_default_language_information($language_code) Gets default language information.

• pll_is_valid_language_code($language_code) Determines if the specified language code is a valid one.

• pll_add_language($language_code, $language_order = 0, &$error_code = 0) Adds a language with the default locale, name and direction.

• pll_is_language_installed($language_code) Determines whether a language is currently installed.