Is it possible to add a site-wide add_rewrite_rule for a multilingual site?

Question

We have a client who has had a brand new site developed by another agency, it is multilingual via some custom fields and some basic logic to display the relevant language via a simple language selector. No plugins involved.

The issue is that the client wants to ensure that their site has SEO that targets the various countries/languages they serve, but the current implementation is not SEO friendly whatsoever due to it being almost locale-adaptive (ie no specific URLs for each language).

Due to the costs, the client is not willing to rebuild the entire site with a plugin such as polylang/WPML etc, and we need to figure out a solution that can almost be dropped in and integrate with their existing fields/translations.

Our agency has proposed the following - we simply create a query variable "lang" that is then prepended to URLs if the selected language is not English, so if French is chosen, site.com/page becomes site.com/fr/page, site.com/blog-category/post becomes site.com/fr/blog-category/post etc. We can detect the query variable and serve the appropriate language.

I have got this partially working with some rewrite rules which I have little experience working with, where lang is a two character query variable:

// Register lang query var
function wf_register_query_vars( $vars ) {
    $vars[] = 'lang';
    return $vars;
}
add_filter( 'query_vars', 'wf_register_query_vars' );

// Add rewrite rules
function wf_rewrite_tag_rule() {
    
    add_rewrite_tag( '%lang%', '([a-z]{2})' );
    
    add_rewrite_rule( '^([a-z]{2})/?$', 'index.php?lang=$matches[1]', 'top' );
    add_rewrite_rule( '^([a-z]{2})/?$', 'index.php?lang=$matches[1]&page_id=12', 'top' ); // Home page
    
    add_rewrite_rule( '^([a-z]{2})/example/?$', 'index.php?lang=$matches[1]&post_type=example', 'top' ); // Example CPT Archive
    add_rewrite_rule( '^([a-z]{2})/example/(.?.+?)(/[0-9]+)?/?$', 'index.php?lang=$matches[1]&example=$matches[2]', 'top' ); // Example CPT
    
    add_rewrite_rule( '^([a-z]{2})/(.?.+?)(/[0-9]+)?/?$', 'index.php?lang=$matches[1]&name=$matches[2]&page=$matches[3]', 'bottom' ); // Posts
    add_rewrite_rule( '^([a-z]{2})/(.?.+?)(/[0-9]+)?/?$', 'index.php?lang=$matches[1]&pagename=$matches[2]&page=$matches[3]', 'top' ); // Pages
    
}
add_action('init', 'wf_rewrite_tag_rule', 10, 0);

With the above, the home page works (it's a static page set as the home page), as do single "post" post-types, and single "example" post types. Pages do not work however (404), and have not yet begun looking into archives, categories, tags, taxonomies etc.

If I swap the order of the bottom two (posts and pages), posts will stop working (404), but pages will start working.

So the question is, is there a way to add a rewrite rule that just covers everything - pages, posts, archives, custom post types, custom taxonomies, pagination etc?

I can handle all other logic (serving 404's for non-existent translations, generating hreflangs, changing WP's locale, html lang attribute etc), it is just the rewrite rules that have left me absolutely stumped!

Answer 1

I don't know if this can be done at once --using maybe an htaccess rule--.

But, I think rebuilding the existing rewrite rules instead of adding new ones can do the trick.

There are filters available for each group of rewrite rules:

post_rewrite_rules
date_rewrite_rules
comments_rewrite_rules
search_rewrite_rules
author_rewrite_rules
page_rewrite_rules
{post_type}_rewrite_rules
{taxonomy}_rewrite_rules

By using 'post_rewrite_rules' filter we can rebuild the post rewrite rules using a code like this:

add_filter('post_rewrite_rules', function($rules){
    $newRules = [];
    /**
     * Loop through the current set of rules and:
     * 1- Prepend the language matching regex
     * 2- Increment the current matched groups by one each
     * 3- Append the matched language to the query as the matched group 1
     * 4- Add the modified rule to the $newRules array
     */
    foreach ($rules as $regex => $query) {
        $newRules['^([a-z]{2})/' . $regex]
            = str_replace(
                ['[2]', '[1]'],
                ['[3]', '[2]'], $query) . '&lang=$matches[1]';
    }
    return $newRules;
});

For the changes to be applied; rewrite rules should be flushed. Also the post permalink should be modified by adding the current language to it, may be using a code like this:

add_filter('post_link', function ($link){
    $lang = get_query_var('lang', 'en');
    return
        str_replace(
            home_url('/'),
            home_url('/' . $lang . '/'),
            $link
        );
});