samlauth.authentication config object must specify a language code

Problem/Motivation

Overall cause: The samlauth module

• has one big config (non-entity) object with a gazillion settings, among which are three translatable labels;
• installs only default minimal config, with no default values for those labels.
• does not set a langcode with the installed config. (Which is fine in this case, AFAICT, but I'm not 100% sure.)

Effects / issues arising from this later:

1. On a single-language EN site, after setting one of those labels, Configuration Inspector reports an error: The samlauth.authentication config object must specify a language code, because it contains translatable values.. (No practical effects observed from this error yet.)
2. On a single-langage non-EN site, a strange bug appears in the config translation screen, until (e.g.?) another module is installed.

I wasn't sure if 1. is an issue with samlauth or Core. I'm pretty sure 2. is a Core issue, but trying to determine what the precise cause was... sent me down a rabbit hole of interrelated Core issues.

Proposed resolution

I don't see any way to 100% effectively solve issue 1, except code that ensures a default language is set during Config::save().

Luckily, the current proposed in-progress MR for #3337864-41: Introduce a dedicated "Configuration default language" different from "Site default language" does just that!

So: postpone.

(I was testing all kinds of permutations to figure out whether I should e.g. always set langcode = en on install, and what effects that would have. But stopped when I saw the related issue.)

Steps to reproduce

Issue 1:

1. drush si; drush en samlauth # works the same, regardless whether config_translation is enabled too

2. drush en config_inspector
   drush config:inspect --only-error --detail samlauth.authentication
   -> no errors

3. drush cset samlauth.authentication login_link_title 'SAML login' # works the same if we swap 2 and 3 commands

4. drush config:inspect --only-error --detail samlauth.authentication
   ➜  🤖 Analyzing…
   
    Legend for Data:
     ✅❓  → Correct primitive type, detailed validation impossible.
     ✅✅  → Correct primitive type, passed all validation constraints.
    --------------------------- --------- ------------- -------------------------------
     Key                         Status    Validatable   Data
    --------------------------- --------- ------------- -------------------------------
     samlauth.authentication     Correct   61%           1 errors
      samlauth.authentication:   Correct   Validatable   The samlauth.authentication
                                                         config object must specify a
                                                         language code, because it
                                                         contains translatable values.
    --------------------------- --------- ------------- -------------------------------
   

5. drush cdel samlauth.authentication login_link_title
   drush config:inspect --only-error --detail samlauth.authentication
   -> no errors

Issue 2:

1. drush si --locale=de; drush en samlauth config_translation config_inspector
   • drush config:get / config:export: no language code (and no defaults for values that were not explicitly set)
   • "Übersetzungen für SAML link titles" tab (admin/config/people/saml/translate) DOES NOT show / is inaccessible.
2. drush cset samlauth.authentication login_link_title 'SAML login' (or do same through UI)
   • drush config:get / config:export: still same as 2a.
   • admin/config/people/saml/translate tab shows, screen shows 2 languages:
     Deutsch (with 'add' button) and Unbekannt (en) (Original) (Note in admin/config/regional/language, only "Deutsch" is enabled)
   • If you at this moment try to add DE translations through the UI, there's a fatal error, because in ConfigTranslationFormBase::buildform()
     

       ( $langcode = $this->mapper->getLangcode() ) === 'en'
     
       ( $this->sourceLanguage = $this->languageManager->getLanguage($langcode) ) === NULL
     

     which causes a fatal PHP TypeError in the $form_element->getTranslationBuild($this->sourceLanguage, ... call.

3. drush en ban # or any module:
   • admin/config/people/saml/translate shows 1 language: "Deutsch (Original)"
   • drush config:inspect --only-error --detail samlauth.authentication: no error
   • drush config:get has langcode=de now. Set by:
     

       public function batchUpdateDefaultConfigLangcodes(array|\ArrayAccess &$context): void {
         $this->localeConfigManager->updateDefaultConfigLangcodes();
     

     (#2905295: Configuration language being overwritten during module install would change this code, but is not progressing.)

At this point I stopped trying to reproduce things + testing whether adding 'langcode = en' on install would solve everything... and didn't try multi-language situations, because I read the code from #3337864-41: Introduce a dedicated "Configuration default language" different from "Site default language".

If that goes through: close this. If not: reevaluate.