From 03c419e70ce8985b2668ae5a96c08ee23965f63f Mon Sep 17 00:00:00 2001
From: Carlos Antonio da Silva <carlosantoniodasilva@gmail.com>
Date: Fri, 13 Feb 2026 10:11:07 -0300
Subject: [PATCH] Only downcase first letter of each auth key, not the entire
 string (#5822)

A previous change #4834 introduced a downcase call to each attribute, so
that it'd fix an invalid grammar issue on some languages like English
that were showing `Email` in the middle of flash message sentences.

However, it caused a bug with German which uses the word `E-Mail` and at
the beginning of the sentence, causing it to be converted to `E-mail`
incorrectly.

The fix here will only downcase the first char of each word, and convert
it back to upcase at the beginning of the sentence, which should work
for both the original fix (English message), and for the new bug (German
 message)

If we end up running into any more of these edge cases with the message,
we might roll it all back and provide a different set of interpolation
values for the original vs downcased translations, so people can use
what makes the most sense for each language without us having to
manually massage these strings.

Fixes #5820
---
 CHANGELOG.md               |  5 +++++
 lib/devise/failure_app.rb  |  4 +++-
 test/failure_app_test.rb   |  5 +++++
 test/support/locale/de.yml | 12 ++++++++++++
 4 files changed, 25 insertions(+), 1 deletion(-)
 create mode 100644 test/support/locale/de.yml

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 38b434e1..e4e2fade 100644
--- a/CHANGELOG.md
+++ b/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
diff --git a/lib/devise/failure_app.rb b/lib/devise/failure_app.rb
index 8222780f..1c9b5865 100644
--- a/lib/devise/failure_app.rb
+++ b/lib/devise/failure_app.rb
@@ -112,7 +112,9 @@ module Devise
         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)
diff --git a/test/failure_app_test.rb b/test/failure_app_test.rb
index b57f4e42..c9e4a56c 100644
--- a/test/failure_app_test.rb
+++ b/test/failure_app_test.rb
@@ -215,6 +215,11 @@ class FailureTest < ActiveSupport::TestCase
 
       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
diff --git a/test/support/locale/de.yml b/test/support/locale/de.yml
new file mode 100644
index 00000000..b6045716
--- /dev/null
+++ b/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."