Cannot read anything other than en in I18n locale

[tmimura39]

Assumption

• Without config.eager_load = true ( I18n.eager_load! don't run ).
• config.i18n.default_locale = :ja ( Locale other than en )

Steps to reproduce

1. Access the Mission Controle Jobs dashboard (/jobs). I18n initial read is performed.
2. Run MyApplicationCode. For example, I18n.l(Time.current)
3. Error occurred Translation missing: ja.time.formats.default (I18n::MissingTranslationData)

Smaller reproduction code

(dev)> I18n.available_locales
=> [:ja, :en]
(dev)> I18n.backend.translations.keys
=> []
(dev)> I18n.with(config: MissionControl::Jobs::I18nConfig.new) { I18n.with_locale(:en) { I18n.l(Time.current) } }
=> "Fri, 23 May 2025 01:01:47 +0900"
(dev)> I18n.backend.translations.keys
=> [:en]
(dev)> I18n.l(Time.current)
(core):5:in '<main>': Translation missing: ja.time.formats.default (I18n::MissingTranslationData)

        raise exception.respond_to?(:to_exception) ? exception.to_exception : exception
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I think it is a bug related to ca09138.

First, I18n file reading is performed only once.
https://github.com/ruby-i18n/i18n/blob/4dddd855039b0c5a0b5b3b2df69783374058a7c9/lib/i18n/backend/simple.rb#L28-L30

Next, non-valid locale files are ignored.
https://github.com/ruby-i18n/i18n/blob/4dddd855039b0c5a0b5b3b2df69783374058a7c9/lib/i18n/backend/simple.rb#L37-L40

In other words, if the first read is performed with available_locales_set set to only en, non-en locales files will be ignored.
And no further reading of locale files will be performed.

Workaround

Run I18n.eager_load! before available_locales_set is updated.

# config/initializers/i18n.rb
Rails.application.config.after_initialize { I18n.eager_load! }

# or config.eager_load = true

(dev)> I18n.available_locales
=> [:ja, :en]
(dev)> I18n.backend.translations.keys
=> []
(dev)> I18n.eager_load!
=> true
(dev)> I18n.backend.translations.keys
=> [:en, :ja]
(dev)> I18n.with(config: MissionControl::Jobs::I18nConfig.new) { I18n.with_locale(:en) { I18n.l(Time.current) } }
=> "Fri, 23 May 2025 01:02:44 +0900"
(dev)> I18n.backend.translations.keys
=> [:en, :ja]
(dev)> I18n.l(Time.current)
=> "2025年05月23日(金) 01時02分52秒 +0900"```