Fix: load :en fallback translations if the I18n backend discarded them [Fix #2987]

[augustocbx]

Motivation / Background

Fixes #2987

When an application restricts I18n.available_locales to a list that does not include :en (e.g. I18n.available_locales = [:de]), calling a generator whose key is missing in the current locale raises:

I18n::MissingTranslationData: Translation missing: en.faker.company.buzzwords

Root cause: the i18n backend (I18n::Backend::Simple#store_translations) silently discards translations for locales that are not in I18n.available_locales when it initializes. So faker's :en data is never stored, and the :en fallback in Faker::Base.translate fails: wrapping the lookup in disable_enforce_available_locales cannot help, because it only affects the lookup — the data was already dropped at load time. This is the line @thdaraujo suspected wasn't working as expected in the issue discussion.

Solution: when the :en fallback lookup fails and the backend has no faker :en data (I18n.exists?('faker', :en) is false), load faker's :en locale files via I18n.backend.load_translations and retry. Properties of this approach:

• Loads only the :en files — it does not pull in all ~60 locales the way the I18n.reload! workarounds from the issue thread do.
• Purely additive — it does not clear or reload the host application's translations (unlike I18n.reload!, which would drop translations an app stored at runtime), and it does not touch I18n.available_locales or enforce_available_locales.
• Runs at most once — after loading, I18n.exists?('faker', :en) is true, so subsequent calls never hit this path again.
• No behavior change otherwise — in a normal setup the guard short-circuits immediately, and keys genuinely missing from :en still raise I18n::MissingTranslationData as before (existing tests cover this).

Both I18n.exists? and load_translations are part of the I18n::Backend::Base API and are available in i18n >= 1.8.11 (the gemspec minimum), including the Chain backend.

Additional information

Console output running the reproduction script from the issue against this branch:

Before:

I18n.available_locales = [:de]
Faker::Company.catch_phrase
# => I18n::MissingTranslationData: Translation missing: en.faker.company.buzzwords

After:

I18n.available_locales = [:de]
Faker::Company.catch_phrase
# => "Public-key human-resource access"
I18n.enforce_available_locales  # => true (restored)
I18n.available_locales          # => [:de] (untouched)
I18n.backend.send(:translations).keys  # => [:de, :en] (only :en added, not all locales)
Faker::Base.translate("faker.nonexistent_key")  # => still raises I18n::MissingTranslationData

The regression test reproduces the issue scenario (it fails with I18n::MissingTranslationData: Translation missing: en.faker.company.buzzwords without the fix). Note the pre-existing test_translation_fallback_without_en_in_available_locales did not catch this because it removes :en after the backend has already initialized and stored the :en data; the bug only manifests when the backend initializes while :en is excluded, hence the I18n.reload! in the new test.

Full suite + RuboCop pass locally: bundle exec rake → 2180 tests, 0 failures, 0 errors; 576 files inspected, no offenses.

Checklist

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