Current entity language and content type language might be different, but they should be identical

+++ b/core/modules/node/src/NodeForm.php
@@ -298,7 +298,8 @@ public function save(array $form, FormStateInterface $form_state) {
         $form_state->setRedirect(
           'entity.node.canonical',
-          ['node' => $node->id()]
+          ['node' => $node->id()],
+          ['language' => $node->language()]
         );

I think here and elsewhere we should use $entity->toUrl() instead, which already adds the language option, at least where possible. That both improves readibility of the code and fixes these language related problems.

Should we add a link relation for this route to the taxonomy term in a hook_entity_type_alter implementation in the forum module?

I'm not necessarily opposed to this, but generally speaking this is not always possible because the route parameters that are necessary for a particular link relation (i.e. Entity::urlRouteParameters()) cannot be extended in any way, so if you need an additional route parameter for a link relation you are out of luck. That's why I generally stick to those link relations defined by the entity itself. In this case it would actually work, though, because just the term is required as a parameter and it would make the affected code quite a bit nicer. So don't feel strongly either way, not really sure.

I thought about running the user entity through EntityRepository::getTranslationFromContext(), but I think that in this case the user entity routes will be different if the content type language changes and I am not sure if we want this as this might happen if the user just visits the e.g. spanish translation of a random entity.

I'm sorry but I can't quite follow that. Can you elaborate what your concerns with ::getTranslationFromContext() are?

Rerolled this one. Also took a look at the patch.

1. +++ b/core/modules/book/src/BookBreadcrumbBuilder.php
   @@ -17,6 +17,13 @@
   +  /**
   +   * The entity manager.
   +   *
   +   * @var \Drupal\Core\Entity\EntityManagerInterface
   +   */
   +  protected $entityManager;
   
   @@ -40,6 +47,7 @@ class BookBreadcrumbBuilder implements BreadcrumbBuilderInterface {
   +    $this->entityManager = $entity_manager;
   

   Let's call this $entityRepository as that's what's actually being used here. We can still assing the entity manager, for now, but that will reduce the things we have to update once we properly deprecate the entity manager.

2. +++ b/core/modules/responsive_image/src/ResponsiveImageStyleForm.php
   @@ -281,13 +281,10 @@ public function save(array $form, FormStateInterface $form_state) {
        if (!$responsive_image_style->hasImageStyleMappings()) {
   ...
   +      $form_state->setRedirectUrl($responsive_image_style->toUrl('edit-form'));
   ...
        else {
   ...
   +      $form_state->setRedirectUrl($responsive_image_style->urlInfo('collection'));
        }
   

   Minor, but now that this is using the same method, the setRedirectUrl() call could be moved outside of the if-else and we could only assing $url = ... in the if-else.

Re-roll of the previous patch.

This update fixes tests (sometimes this required changing tests logic, because on entity translation save the redirect is now to the translation, not to the original entity), sometimes changing the patch itself (eg, for the user, whih is also a content entity, logic on login should be different - language should be kept), sometimes removing deprecations in the patch, or changing mocks and expectations for unit tests.

Another attempt to fix all tests. And, are we sure that on user login and redirect to user profile, we want to keep content language, not the interface language?

Simple reroll for 8.8.x

Re-roll patch #33 on 8.9

Now fixed issues partially working on rest of issue once fix. I will update here.

Re-roll for 9.2.
Not getting any clue how to fix (FATAL Drupal\Tests\language\Functional\LanguageUrlRewritingTest: test runner returned a non-zero error code (2).) error if anybody have any idea let me know. Thanks

Rerolled for 9.5 and fixed some phpcs errors.

The IS appears to be incomplete.

For the issue summary.