Polylang missing date time translation

After investigating the problem, it seems that the wordpress function human_time_diff() in formatting.php doens′t return translated strings (although the _n() function is called).

I tried to override the locale variable and added a text-domain, but that doesn′t help, so I ended up in translating it in my themes functions manually.

add_filter('human_time_diff', 'new_human_time_diff', 10, 2);
function new_human_time_diff($from, $to) {

    // remove filter to prevent the loop
    remove_filter('human_time_diff', 'new_human_time_diff');

    $locale = get_locale();

    $timediff = human_time_diff($from, $to);
    if($locale == "en_US"){
        return $timediff;
     } else {
        $translate_from_plural = array("seconds", "mins", "days", "hours", "months", "weeks", "years");
        $translate_from_singular = array("second", "min", "day", "hour", "month", "week", "year");
        If ($locale == "de_DE"){
            $translate_to_plural = array("Sekunden", "Minuten", "Tagen", "Stunden", "Monaten", "Wochen", "Jahren");
            $translate_to_singluar = array("Sekunde", "Minute", "Tag", "Stunde", "Monat", "Woche", "Jahr");
        } elseif ($locale == "ja"){
            $translate_to_plural = array("秒", "分", "日々", "時間", "月", "週", "年");
            $translate_to_singluar = array("秒", "分", "日", "時間", "月", "週間", "年");
        }
        $timediff = str_replace($translate_from_singular,$translate_to_singluar,str_replace($translate_from_plural, $translate_to_plural, $timediff));
    
    }
    // restore the filter
    add_filter( 'human_time_diff', 'new_human_time_diff', 10, 2);     

    return $timediff;
}