Enabling and configuring content language negotiation does not work at once

core/modules/language/src/LanguageNegotiator.php:

  function saveConfiguration($type, $enabled_methods) {
    $definitions = $this->getNegotiationMethods();
    $default_types = $this->languageManager->getLanguageTypes();
    [...]

$default_types contains only language_interface yet.

Ok, gotcha.

I'm not sure if this is what we expect, but this is how language.types.yml looks like after saving some config:

all:                                                                                                                                                                     
  - language_interface
  - language_content
  - language_url
configurable:
  - language_interface
  - language_content
negotiation:
  language_content:
    enabled:
      language-url: -8
      language-user: -4
      language-browser: -2
      language-selected: 12
    method_weights:
      language-url: -8
      language-session: -6
      language-user: -4
      language-browser: -2
      language-interface: 9
      language-selected: 12
  language_url:
    enabled:
      language-url: 0
      language-url-fallback: 1
  language_interface:
    enabled:
      language-url: -8
      language-browser: -2
      language-selected: 12
    method_weights:
      language-url: -8
      language-session: -6
      language-user: -4
      language-browser: -2
      language-user-admin: 10
      language-selected: 12

The test is checking with in_array, but should use array_key_exists.

Sure, done that.

+++ b/core/modules/language/src/LanguageNegotiator.php
@@ -264,6 +264,8 @@ public function isNegotiationMethodEnabled($method_id, $type = NULL) {
+    // As configurable language types might have changed, we reset the cache.
+    $this->languageManager->reset();

Why doesn't the code that changes the configurable language types reset this cache?

We are already resetting the language manager (even clearing the negotiator manager cache) on purgeConfiguration() and updateConfiguration()... so why not in saveConfiguration()? Otherwise, the form will know too much about the language manager, and even can cause issues if reproducing this via API.

I agree with @penyaskito putting the cache clear on the form layer feels wrong - having it in LanguageNegotiator::saveConfiguration() feels right - if anything calls that API, Drush, form this will reset the language manager and the configuration we change the system as expected once saved.

@penyaskito wanted to assign this to @catch

Something is still niggling but it's not concrete, and I agree this shouldn't be done in the form, so this seems OK.

Committed/pushed to 8.0.x, thanks!