how to localize the number of wordpress post views?

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The problem is most probably, that you don’t actually call the filter. WordPress does this not automatically for you.

If you want to create a custom filter you have to manually call apply_filters, so that the added filters get executed:

function the_views($postID){
    $count_key = 'views';
    $count = get_post_meta($postID, $count_key, true);
    if($count==''){
        delete_post_meta($postID, $count_key);
        add_post_meta($postID, $count_key, '0');
        $count = 0;
    }

    // use apply_filters to call the filter
    return sprintf($count == 0 ? '%s View' : '%s বার', 
        apply_filters('the_views', $count));

    // better alternative: use the WordPress translation API:
    //return sprintf(_n('%s View', '%s Views', $count), 
        //apply_filters('the_views', $count));
}

I have modified the code a bit so the digit “0” gets also translated/converted. I have also added a suggestion at the end to use the WP translation API. There’s a bit more effort required to translate it that way but it would bring some benefits (you could translate it to other languages without code-changes and the result would be proper pluralized).