add_action( 'init', 'rule' );

function rule() {

        add_rewrite_rule(
            '^(en|fr|de|ru)/(.?.+?)/?$',
            'index.php?lang=$matches[1]&pagename=$matches[2]',
            'top'
        );    
}

The code above is part of all my code. First of all, I must say, my code works on everything else. (Date, Category, Label, etc.)

Even the code above works fine for "post_type=page" ie page type. However, if "post_type=post" is entered,

It automatically redirects "localhost/en/hello-world" to "localhost/hello-world". But it works fine if a page like "localhost/en/contact" is entered.

• "Localhost/en/category/food"
• "Localhost/en/2019/08/28"
• "Localhost/en/tag/food"

I don't make sense of why a "post" doesn't work when thats are working properly.

Who knows, maybe I should write separate rules for "page" and "post". and!

I tried:

    add_rewrite_rule(
        '^(en|fr|de|ru)/(.?.+?)/?$',
        'index.php?lang=$matches[1]&pagename=$matches[2]&post_type=posts',
        'top'
    );

    add_rewrite_rule(
        '^(en|fr|de|ru)/(.?.+?)/?$',
        'index.php?lang=$matches[1]&pagename=$matches[2]',
        'top'
    );

but again I didn't get a result. I don't know if I add any "custom_post_type" to the same problem.

I am looking forward to your thoughts and solutions. Thanks.