Only downcase first letter of each auth key, not the entire string

CHANGELOG.md

@@ -1,3 +1,8 @@
+### Unreleased
+
+* bug fixes
+  * Fix translation issue with German `E-Mail` on invalid authentication messages caused by previous fix for incorrect grammar [#5822](https://github.com/heartcombo/devise/pull/5822)
+
 ### 5.0.0 - 2026-01-23
 
 no changes

lib/devise/failure_app.rb

@@ -112,7 +112,9 @@ def i18n_message(default = nil)
         options[:default] = [message]
         auth_keys = scope_class.authentication_keys
         human_keys = (auth_keys.respond_to?(:keys) ? auth_keys.keys : auth_keys).map { |key|
-          scope_class.human_attribute_name(key).downcase
+          # TODO: Remove the fallback and just use `downcase_first` once we drop support for Rails 7.0.
+          human_key = scope_class.human_attribute_name(key)
+          human_key.respond_to?(:downcase_first) ? human_key.downcase_first : human_key[0].downcase + human_key[1..]
         }
         options[:authentication_keys] = human_keys.join(I18n.t(:"support.array.words_connector"))
         options = i18n_options(options)

test/failure_app_test.rb

@@ -215,6 +215,11 @@ def call_failure(env_params = {})
 
       assert_equal 'Email ou senha inválidos.', @request.flash[:alert]
       assert_equal 'http://test.host/users/sign_in', @response.second["Location"]
+
+      call_failure('warden' => OpenStruct.new(message: :invalid), 'warden.options' => { locale: :de })
+
+      assert_equal 'E-Mail oder Passwort ist ungültig.', @request.flash[:alert]
+      assert_equal 'http://test.host/users/sign_in', @response.second["Location"]
     end
 
     test 'uses the proxy failure message as string' do

test/support/locale/de.yml

@@ -0,0 +1,12 @@
+de:
+  activerecord:
+    attributes:
+      user:
+        email: E-Mail
+  mongoid:
+    attributes:
+      user:
+        email: E-Mail
+  devise:
+    failure:
+      invalid: "%{authentication_keys} oder Passwort ist ungültig."