Create a custom I18n config class to set available locales

Without interferring with the host application's I18n configuration.
I18n's available_locales unfortunately is not thread-safe so we can't
just change that, but I18n.config is thread-safe [1], so we can use our own
there, that just relies on all I18n defaults except for available
locales.

[1] https://github.com/ruby-i18n/i18n/blob/3b65f6548245411bc9802f5a547954d370b57821/lib/i18n.rb#L56-L64

app/controllers/mission_control/jobs/application_controller.rb

@@ -12,8 +12,19 @@ class MissionControl::Jobs::ApplicationController < MissionControl::Jobs.base_co
   include MissionControl::Jobs::ApplicationScoped, MissionControl::Jobs::NotFoundRedirections
   include MissionControl::Jobs::AdapterFeatures
 
+  around_action :set_current_locale
+
   private
     def default_url_options
       { server_id: MissionControl::Jobs::Current.server }
     end
+
+    def set_current_locale(&block)
+      @previous_config = I18n.config
+      I18n.config = MissionControl::Jobs::I18nConfig.new
+      I18n.with_locale(:en, &block)
+    ensure
+      I18n.config = @previous_config
+      @previous_config = nil
+    end
 end

app/helpers/mission_control/jobs/dates_helper.rb

@@ -1,15 +1,19 @@
 module MissionControl::Jobs::DatesHelper
   def time_distance_in_words_with_title(time)
-    tag.span time_ago_in_words(time, include_seconds: true), title: "Since #{time.to_fs(:long)}"
+    tag.span time_ago_in_words_with_default_options(time), title: "Since #{time.to_fs(:long)}"
   end
 
   def bidirectional_time_distance_in_words_with_title(time)
     time_distance = if time.past?
-      "#{distance_of_time_in_words_to_now(time, include_seconds: true)} ago"
+      "#{time_ago_in_words_with_default_options(time)} ago"
     else
-      "in #{distance_of_time_in_words_to_now(time, include_seconds: true)}"
+      "in #{time_ago_in_words_with_default_options(time)}"
     end
 
     tag.span time_distance, title: time.to_fs(:long)
   end
+
+  def time_ago_in_words_with_default_options(time)
+    time_ago_in_words(time, include_seconds: true, locale: :en)
+  end
 end

lib/mission_control/jobs/i18n_config.rb

@@ -0,0 +1,5 @@
+class MissionControl::Jobs::I18nConfig < ::I18n::Config
+  def available_locales
+    [ :en ]
+  end
+end

test/controllers/jobs_controller_test.rb

@@ -32,7 +32,7 @@ class MissionControl::Jobs::JobsControllerTest < ActionDispatch::IntegrationTest
     assert_response :ok
 
     assert_select "tr.job", 2
-    assert_select "tr.job", /AutoRetryingJob\s+Enqueued less than a minute ago\s+default/
+    assert_select "tr.job", /AutoRetryingJob\s+Enqueued less than 5 seconds ago\s+default/
 
     get mission_control_jobs.application_job_url(@application, job.job_id)
     assert_response :ok
@@ -104,16 +104,31 @@ class MissionControl::Jobs::JobsControllerTest < ActionDispatch::IntegrationTest
     end
   end
 
-  test "get jobs and job details the default locale is set to another language than English" do
-    I18n.available_locales = %i[en nl]
+  test "get jobs and job details when the default locale is set to another language than English" do
+    previous_locales, I18n.available_locales = I18n.available_locales, %i[ en nl ]
 
     DummyJob.set(wait: 3.minutes).perform_later
 
     I18n.with_locale(:nl) do
       get mission_control_jobs.application_jobs_url(@application, :scheduled)
       assert_response :ok
 
-      assert_select "tr.job", /DummyJob\s+Enqueued less than a minute ago\s+queue_1\s+in 3 minutes/
+      assert_select "tr.job", /DummyJob\s+Enqueued less than 5 seconds ago\s+queue_1\s+in 3 minutes/
     end
+  ensure
+    I18n.available_locales = previous_locales
+  end
+
+  test "get jobs and job details when English is not included among the locales" do
+    previous_locales, I18n.available_locales = I18n.available_locales, %i[ es nl ]
+
+    DummyJob.set(wait: 3.minutes).perform_later
+
+    get mission_control_jobs.application_jobs_url(@application, :scheduled)
+    assert_response :ok
+
+    assert_select "tr.job", /DummyJob\s+Enqueued less than 5 seconds ago\s+queue_1\s+in 3 minutes/
+  ensure
+    I18n.available_locales = previous_locales
   end
 end