How to alter and rewritte current path programmatically in Drupal 8?
I was able to achieve what I wanted to do by using PathProcessor service.

Here is my services.yml
  mymodule.path_processor:
    class: Drupal\mymodule\PathProcessor\PathProcessor
    arguments: ['@language_manager', '@config.factory', '@path.alias_manager']
    tags:
      - { name: path_processor_inbound, priority: 200 }


And my code in my PathProcessor.php class :

/**
 * Processes the inbound path using path alias lookups.
 */
class PathProcessorCustom implements InboundPathProcessorInterface, OutboundPathProcessorInterface {

    protected $language_manager;
    protected $configFactory;
    protected $aliasManager;

    public function __construct(LanguageManagerInterface $language_manager, ConfigFactoryInterface $config_factory, AliasManagerInterface $alias_manager) {
        $this-&gt;languageManager = $language_manager;
        $this-&gt;configFactory = $config_factory;
        $this-&gt;aliasManager = $alias_manager;
    }

    public function processInbound($path, Request $request) {
        if (!strpos($path, '/admin/')) {
            $path = $this-&gt;removeISOCode($path);
        }

        return $path;
    }

    public function processOutbound($path, &amp;$options = array(), Request $request = NULL, BubbleableMetadata $bubbleable_metadata = NULL) {
        if (!strpos($path, '/admin/')) {
            $path = $this-&gt;insertISOCode($path);
        }

        return $path;
    }

    protected function removeISOCode($path) {
        $langcode = $this-&gt;languageManager-&gt;getDefaultLanguage()-&gt;getId();
        $isocodes = $this-&gt;getISOCodeMapping();
        $path_items = explode('/', $path);
        if (isset($path_items[1]) &amp;&amp; strlen($path_items[1]) == 2 &amp;&amp; in_array($path_items[1], $isocodes[$langcode])) {
            $config_negotiation_url = $this-&gt;configFactory-&gt;get('language.negotiation')-&gt;get('url');
            if (!in_array($path_items[1], $config_negotiation_url['prefixes'])) {
                array_splice($path_items, 1, 1);
                if (count($path_items) == 1) {
                    $config_system_site = $this-&gt;configFactory-&gt;get('system.site')-&gt;get('page');
                    $path = $config_system_site['front'];
                }
                else {
                    $path = implode('/', $path_items);
                    $path = $this-&gt;aliasManager-&gt;getPathByAlias($path, 'en');
                }
            }
        }

        return $path;
    }

    protected function insertISOCode($path) {
        $path_items = explode('/', $path);
        if (isset($path_items[1]) &amp;&amp; strlen($path_items[1]) == 2 &amp;&amp; $path_items[1] == 'gb') {
            $config_negotiation_url = $this-&gt;configFactory-&gt;get('language.negotiation')-&gt;get('url');
            if(!in_array($path_items[1], $config_negotiation_url['prefixes'])) {
                $path_items[1] = $language;
            }
        }

        $path = implode('/', $path_items);

        return $path;
    }

    protected function getISOCodeMapping() {
        return [
            'en' =&gt; ['gb', 'us'],
            'fr' =&gt; ['fr', 'be']
        ];
    }

}


What I do is I decompose URL and remove what I know I don't need and map the URL to an actual content URL. Otherwise, redirect to the default 404.