Redmine

It seems that pluralization rules(config/locales/*.rb) is also loaded when loading the locale files located in I18n.load_path(source:/trunk/config/application.rb#L42).

https://github.com/ruby-i18n/i18n/wiki/Pluralizations

One can ship pluralizers (i.e. lambdas that implement locale specific pluralization algorithms) as part of any Ruby translation file anywhere in the I18n.load_path. The implementation expects to find them with the key :pluralize in a (newly invented) translation metadata namespace :i18n.

Therefore, it seems that you can't set pluralization rules in the middle like I18nTest#test_custom_pluralization_rules.
I have confirmed that the tests succeed with the following patch.

diff --git a/test/unit/lib/redmine/i18n_test.rb b/test/unit/lib/redmine/i18n_test.rb
index 4e15ae56b7..f6e7a804d1 100644
--- a/test/unit/lib/redmine/i18n_test.rb
+++ b/test/unit/lib/redmine/i18n_test.rb
@@ -257,11 +257,13 @@ class Redmine::I18nTest < ActiveSupport::TestCase
   end

   def test_custom_pluralization_rules
+    pluralizers = I18n.backend.instance_variable_get(:@pluralizers)
+    I18n.backend.instance_variable_set(:@pluralizers, nil)
     I18n.backend.store_translations :pt, i18n: {plural: {rule: ->(n) {[0, 1].include?(n) ? :one : :other }}}
     I18n.backend.store_translations :pt, apples: {one: 'one or none', other: 'more than one'}
     assert_equal 'one or none', ll(:pt, :apples, count: 0)
     assert_equal 'more than one', ll(:pt, :apples, count: 2)
   ensure
-    I18n.reload!
+    I18n.backend.instance_variable_set(:@pluralizers, pluralizers)
   end
 end

Committed the fix as a part of #36396. Thank you.