Fixes #4000 - Unable to upgrade to Zammad 5.1.

app/models/translation/synchronizes_from_po.rb

@@ -95,7 +95,10 @@ module Translation::SynchronizesFromPo
       return ['i18n/zammad.pot'] if locale.eql? 'en-us'
 
       files = Dir.glob "i18n/*.#{locale}.po", base: Rails.root
-      raise "No translation found for locale '#{locale}'." if files.exclude?("i18n/zammad.#{locale}.po")
+      if files.exclude?("i18n/zammad.#{locale}.po")
+        Rails.logger.error "No translation found for locale '#{locale}'."
+        return []
+      end
 
       [
         files.delete("i18n/zammad.#{locale}.po"),

db/migrate/20220308072741_rename_tw_locale.rb

@@ -0,0 +1,21 @@
+# Copyright (C) 2012-2022 Zammad Foundation, https://zammad-foundation.org/
+
+class RenameTwLocale < ActiveRecord::Migration[6.0]
+  # Copied from db/migrate/20180502015927_issue_1219_zhtw_locale_typo.rb as this
+  #   needed to be re-executed.
+  def change
+    return if !Setting.exists?(name: 'system_init_done')
+
+    if Locale.exists?(locale: 'zh-tw')
+      Locale.find_by(locale: 'zj-tw')&.destroy
+    else
+      Locale.find_by(locale: 'zj-tw')&.update(locale: 'zh-tw')
+    end
+
+    Translation.where(locale: 'zj-tw')&.update_all(locale: 'zh-tw') # rubocop:disable Rails/SkipsModelValidations
+    User.where('preferences LIKE ?', "%\nlocale: zj-tw\n%").each do |u|
+      u.preferences[:locale] = 'zh-tw'
+      u.save
+    end
+  end
+end

spec/models/translation/translation_synchronizes_from_po_spec.rb

@@ -7,8 +7,14 @@ RSpec.describe Translation do
   context 'when getting the list of files for a locale' do
 
     context 'when a locale is nonexistent' do
-      it 'throws' do
-        expect { described_class.po_files_for_locale('nonexisting-locale') }.to raise_error "No translation found for locale 'nonexisting-locale'."
+      it 'logs an error' do
+        allow(Rails.logger).to receive(:error)
+        described_class.po_files_for_locale('nonexisting-locale')
+        expect(Rails.logger).to have_received(:error).with("No translation found for locale 'nonexisting-locale'.")
+      end
+
+      it 'returns an empty array' do
+        expect(described_class.po_files_for_locale('nonexisting-locale')).to eq([])
       end
     end
 
@@ -41,8 +47,14 @@ RSpec.describe Translation do
 
   context 'when getting po strings for a locale' do
     context 'when getting strings for a nonexistent locale' do
-      it 'throws' do
-        expect { described_class.strings_for_locale('nonexisting-locale') }.to raise_error "No translation found for locale 'nonexisting-locale'."
+      it 'logs an error' do
+        allow(Rails.logger).to receive(:error)
+        described_class.strings_for_locale('nonexisting-locale')
+        expect(Rails.logger).to have_received(:error).with("No translation found for locale 'nonexisting-locale'.")
+      end
+
+      it 'returns an empty array' do
+        expect(described_class.strings_for_locale('nonexisting-locale')).to eq({})
       end
     end