Language switcher is missing.
After including the file like this in main theme (i'm not using child theme) functions file like this (i'm using predefined constants with full directory addresses):
require_once(PN_LIBRARIES_DIR . '/customizer-polylang/customizer-polylang.php');
the language switcher is missing.

[UPDATE] In Console i get this error:
Loading failed for the <script> with source “http://localhost/project/back-end/wp-content/themes/project/js/customizer-polylang.js?ver=1438771592”. customize.php:1602:1
ReferenceError: PSPolyLang is not defined
If I change 246 line in customizer-polylang.php to also use constant with full directory URL like so:
$src = PN_LIBRARIES_URL . '/customizer-polylang/js/customizer-polylang.js';
Then I get it working.
Is this issue because I'm using it in main theme and not in child theme?
No, it should work for both main and child themes. I'll look into it (thought I'd fixed this, have to compare the code here with the one I use in production)
Well, just in case the full adrresses for DIRs and URLs I've predefined look like this:
// Directory addresses
define("PN_BASE_DIR", dirname(dirname(__FILE__)));
define("PN_INCLUDES_DIR", PN_BASE_DIR . '/includes');
define("PN_LIBRARIES_DIR", PN_BASE_DIR . '/libraries');
define("PN_ASSETS_DIR", PN_BASE_DIR . '/assets');
define("PN_LANGUAGE_DIR", PN_BASE_DIR . '/languages');
define("PN_TEMPLATES_DIR", PN_BASE_DIR . '/templates');
// URL addresses
define("PN_BASE_URL", get_stylesheet_directory_uri());
define("PN_INCLUDES_URL", PN_BASE_URL . '/includes');
define("PN_LIBRARIES_URL", PN_BASE_URL . '/libraries');
define("PN_ASSETS_URL", PN_BASE_URL . '/assets');
define("PN_LANGUAGE_URL", PN_BASE_URL . '/languages');
define("PN_TEMPLATES_URL", PN_BASE_URL . '/templates');
And the final output, looks like this (i double checked and I think it's something to do with using main theme and not child theme:
string(80) "C:\Users\user\Projects\project\back-end\wp-content\themes\project/libraries"
string(69) "http://localhost/project/back-end/wp-content/themes/project/libraries"
So.. in the functions.php I call your library like this:
require_once(PN_LIBRARIES_DIR . '/customizer-polylang/customizer-polylang.php');
and changed 246 line in customizer-polylang.php to this:
$src = PN_LIBRARIES_URL . '/customizer-polylang/js/customizer-polylang.js';
And thanks for free solution. Really appreciate it.