I’ve found a solution:
on kbe-template-functions.php file,
instead of this:
<form role="search" method="get" id="searchform" class="clearfix" action="<?php echo home_url( '/' ); ?>" autocomplete="off">
<input type="text" onfocus="if (this.value == '<?php _e( 'Search Articles...', 'wp-knowledgebase' ); ?>') {this.value = '';}" onblur="if (this.value == '') {this.value = '<?php _e( 'Search Articles...', 'wp-knowledgebase' ); ?>';}" value="<?php _e( 'Search Articles...', 'wp-knowledgebase' ); ?>" name="s" id="s" />
paste this:
<form role="search" method="get" id="searchform" class="clearfix" action="<?php echo esc_url(home_url('/') . pll_current_language() . '/'); ?>" autocomplete="off">
<input type="text" onfocus="if (this.value == '<?php pll_e( 'Search Articles...', 'wp-knowledgebase' ); ?>') {this.value = '';}" onblur="if (this.value == '') {this.value = '<?php pll_e( 'Search Articles...', 'wp-knowledgebase' ); ?>';}" value="<?php pll_e( 'Search Articles...', 'wp-knowledgebase' ); ?>" name="s" id="s" />
and on kbe-core-functions.php file,
instead of this:
jQuery('#live-search #s').liveSearch({url: '<?php echo home_url(); ?>/?ajax=on&post_type=kbe_knowledgebase&s='});
paste this:
jQuery('#live-search #s').liveSearch({url: '<?php echo esc_url(home_url('/') . pll_current_language() . '/'); ?>/?ajax=on&post_type=kbe_knowledgebase&s='});