Excluded paths are not valid for multilingual sites that have language prefix.

Looks ok, just a few nitpicks:

1. +++ b/modules/tome_static/src/EventSubscriber/ExcludePathSubscriber.php
   @@ -71,6 +71,17 @@ class ExcludePathSubscriber implements EventSubscriberInterface {
   +    if (\Drupal::languageManager()->isMultilingual() && \Drupal::config('language.types')->get('negotiation.language_interface.enabled.language-url')) {
   

   Services should be injected.

2. +++ b/modules/tome_static/src/EventSubscriber/ExcludePathSubscriber.php
   @@ -71,6 +71,17 @@ class ExcludePathSubscriber implements EventSubscriberInterface {
   +      $languageNegotiation = $config->get('url.prefixes');
   

   The variable name doesn't match the data. Core tends to have: $prefixes = $config->get('url.prefixes');

3. +++ b/modules/tome_static/src/EventSubscriber/ExcludePathSubscriber.php
   @@ -71,6 +71,17 @@ class ExcludePathSubscriber implements EventSubscriberInterface {
   +        foreach ($languageNegotiation as $lang) {
   

   $lang should be $langcode.

@joachim,
Adding patch with variable name changes. Looked into dependency injection point but getExcludedPaths() is a static method and we won't be able to use $this inside static method. If we make it non-static, then we will have to modify all code where this method is being called.

Your feedback is appreciable.

Ah I hadn't seen it was static. That's because it's called like this:

$paths = ExcludePathSubscriber::getExcludedPaths();

from StaticGenerator. Which is just ICK but out of scope for this issue.

I couldn't find anywhere in core that actually documents this, but AFAICT the prefixes config is $langcode => $prefix.

So the loop variable should be $prefix.

Also, it can be empty! In that case, you'd be incorrectly adding a second slash at the front of the path.

Attaching new patch with your feedback.

Nearly there! :)

+++ b/modules/tome_static/src/EventSubscriber/ExcludePathSubscriber.php
@@ -71,6 +71,17 @@ class ExcludePathSubscriber implements EventSubscriberInterface {
+          $excluded_paths[] = !empty($prefix) ? "/" . $prefix . $path : $path;

If $prefix is empty, there's no need to add $path, it's already in the array.

Also, nitpick, but the / can be single-quoted.

Another nitpick: would be nice to have a comment at the top of the section to explain we're adding language-prefixed versions of all the paths.

Adding new patch with addressed feedback.

LGTM!