Fallback to english for untranslated text

**Why**: So that users using the site in other languages will not see
NOT TRANSLATED AT.

**How**: Patch `I18n.translate` and `I18n.t` such that when it is about
to return "NOT TRANSLATED AT", it instead calls back to the I18n
backend for the same translation in english.

config/initializers/i18n_english_fallback.rb

@@ -0,0 +1,21 @@
+I18n.module_eval do
+  class << self
+    def translate(*args)
+      original_translation = super(*args)
+      return original_translation unless original_translation == 'NOT TRANSLATED YET'
+      fallback_to_english(*args)
+    end
+
+    alias_method :t, :translate
+
+    private
+
+    def fallback_to_english(*args)
+      key = args.shift
+      options = args.last.is_a?(Hash) ? args.last : {}
+      options.delete(:locale)
+
+      config.backend.translate(:en, key, options)
+    end
+  end
+end

spec/config/initializers/i18n_english_fallback.rb

@@ -0,0 +1,73 @@
+require 'rails_helper'
+
+describe I18n do
+  let(:translation_key) { 'asdf.qwert.1234' }
+  let(:english_translation) { 'this is some text' }
+  let(:spanish_translation) { 'NOT TRANSLATED YET' }
+  let(:local_argument) { :en }
+
+  before do
+    allow(I18n.config.backend).to receive(:translate).
+      with(:es, translation_key, {}).
+      and_return(spanish_translation)
+    allow(I18n.config.backend).to receive(:translate).
+      with(:en, translation_key, {}).
+      and_return(english_translation)
+  end
+
+  after do
+    I18n.locale = :en
+  end
+
+  describe '#translate' do
+    context 'with non-english locale' do
+      context 'when the requested string is untranslated' do
+        it 'should return the english translation with a locale argument' do
+          expect(I18n.t('asdf.qwert.1234', locale: :es)).to eq(english_translation)
+        end
+
+        it 'should return the english translation with a global locale' do
+          I18n.locale = :es
+
+          expect(I18n.t('asdf.qwert.1234')).to eq(english_translation)
+        end
+      end
+
+      context 'when the requested string is translated' do
+        let(:spanish_translation) { 'esto es un texto' }
+
+        it 'should return the non-english translation with a locale argument' do
+          expect(I18n.t('asdf.qwert.1234', locale: :es)).to eq(spanish_translation)
+        end
+
+        it 'should return the non-english translation with a global locale' do
+          I18n.locale = :es
+
+          expect(I18n.t('asdf.qwert.1234')).to eq(spanish_translation)
+        end
+      end
+    end
+
+    context 'with english locale' do
+      it 'should return the english translation with a locale argument' do
+        expect(I18n.t('asdf.qwert.1234', locale: :en)).to eq(english_translation)
+      end
+
+      it 'should return the english translation with a global locale' do
+        expect(I18n.t('asdf.qwert.1234')).to eq(english_translation)
+      end
+    end
+
+    context 'when english translation is "NOT TRANSLATED YET"' do
+      let(:english_translation) { 'NOT TRANSLATED YET' }
+
+      it 'does not recurse with a locale argument' do
+        expect(I18n.t('asdf.qwert.1234', locale: :en)).to eq('NOT TRANSLATED YET')
+      end
+
+      it 'does not recurse without a locale argument' do
+        expect(I18n.t('asdf.qwert.1234')).to eq('NOT TRANSLATED YET')
+      end
+    end
+  end
+end