Reset I18n.locale to not affect other specs

Otherwise spec/models/enterprise_spec.rb fails if run after this.

spec/helpers/i18n_helper_spec.rb

@@ -3,6 +3,16 @@ require 'spec_helper'
 describe I18nHelper do
   let(:user) { create(:user) }
 
+  # In the real world, the helper is called in every request and sets
+  # I18n.locale to the chosen locale or the default. For testing purposes we
+  # have to restore I18n.locale for unit tests that don't call the helper, but
+  # rely on translated strings.
+  around do |example|
+    locale = I18n.locale
+    example.run
+    I18n.locale = locale
+  end
+
   context "as guest" do
     before do
       allow(helper).to receive(:spree_current_user) { nil }