NodeTranslationUITest should use API to set up language

    $edit = ['field_storage[subform][cardinality_number]' => 2];
    $this->drupalGet('admin/structure/types/manage/article/fields/node.article.field_image');
    $this->submitForm($edit, 'Save');

    // Make the image field non-translatable.
    $edit = ['settings[node][article][fields][field_image]' => FALSE];
    $this->drupalGet('admin/config/regional/content-language');
    $this->submitForm($edit, 'Save configuration');

    $edit['use_admin_theme'] = FALSE;
    $this->drupalGet('admin/appearance');
    $this->submitForm($edit, 'Save configuration');

Could these be converted too?

LGTM

+++ b/core/modules/node/tests/src/Functional/NodeTranslationUITest.php
@@ -258,21 +263,19 @@ public function testTranslationLinkTheme() {
+    $this->container->get('router.builder')->rebuild();

Why do we need to do a router rebuild here but not in the earlier case? This sort of thing makes me take pause about these conversions because it's not clear why we have to do this now when we didn't before.

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> Why do we need to do a router rebuild here but not in the earlier case? This sort of thing makes me take pause about these conversions because it's not clear why we have to do this now when we didn't before.

Let's try without it :)
Part of the problem with these conversions is that the language system's API is complicated and not well-documented.

Why do we need to do a router rebuild here but not in the earlier case

Because changing node.settings.use_admin_theme requires a route rebuild. Before this was done via the form submission handler.

Some of the changes from the patch were already implemented or no longer applied.

> Because changing node.settings.use_admin_theme requires a route rebuild

Let's add a comment to say that. Otherwise someone in the future will wonder about that all over again.

Done