Snippet for StackOverflow - https://stackoverflow.com/questions/65479448/search-form-redirect-to-home-page-after-you-search-with-cyrillic-letters-only/65481297#65481297

function.php

<?php
const CYR = array('а','б','в','г','д','ђ','е','ж','з','и','ј','к','л','љ','м','н','њ','о','п','р','с','т','ћ','у','ф','х','ц','ч','џ','ш','А','Б','В','Г','Д','Ђ','Е','Ж','З','И','Ј','К','Л','Љ','М','Н','Њ','О','П','Р','С','Т','Ћ','У','Ф','Х','Ц','Ч','Џ','Ш');

const LAT = array('a','b','v','g','d','đ','e','ž','z','i','j','k','l','lj','m','n','nj','o','p','r','s','t','ć','u','f','h','c','č','dž','š','A','B','V','G','D','Đ','E','Ž','Z','I','J','K','L','LJ','M','N','NJ','O','P','R','S','T','Ć','U','F','H','C','Č','DŽ','Š');

function cyr_to_latin( $string ) {
    return str_replace(CYR, LAT, $string);
}

function latin_to_cyr( $string ) {
    return str_replace(LAT, CYR, $string);
}

function searchfilter($query) {
 		$search_term = get_search_query();
 		$latin_conversion = cyr_to_latin($search_term);
 		$urldecode = urldecode($latin_sanitize);
 		
    if ($query->is_search && !is_admin() ) {
        $query->set('post_type',array('post','page'));
        $query->set('s',$urldecode);
    }
 
return $query;
}
add_filter('pre_get_posts','searchfilter');