Adding the switcher with PHP

Put this in your theme where you want the button, for example in header.php:

<?php if ( class_exists( 'CLS_Pro' ) ) echo CLS_Pro::trigger(); ?>

With options:

<?php
if ( class_exists( 'CLS_Pro' ) ) {
    echo CLS_Pro::trigger( [
        'style'        => 'minimal',   // pill | minimal | classic | card | icon_only
        'mobile_style' => 'icon_only', // optional
        'show_arrow'   => false,       // optional
    ] );
}

The class_exists() check keeps your theme working if the plugin is ever turned off. The language window is added to the page automatically. See Developers → PHP API for everything else.