DeathAngelDaemon on "[Plugin: Polylang] How to add swichers at the very TOP of site"

That looks like the Plugin "polylang" is de-activated.
Maybe you can test, whether the plugin is activated.

<?php
if( class_exists('Polylang') && function_exists('pll_the_languages') ) {
    return 'Yeah, the plugin is activated and should be displayed!';
} else {
    return 'FAIL!';
}
?>

If you get the second return, please have a look at your pluginlist and activate polylang if it is not. Otherwise there is maybe another error.
Where have you put in the pll_the_languages(); function?