feat(i18n): clarify captcha challenge labels

- harmonize challenge prompts across supported locales
- reuse shared helper to reduce duplicated translations
- ensure login flow references updated label key

Signed-off-by: Qi HU <github@spcsky.com>

lang/de/lang_functions.php

@@ -41,6 +41,7 @@ $lang_functions = array
 	'std_you_will_get' => 'You will get',
 	'std_by' => 'By',
 	'row_security_image' => "Security Image:",
+	'row_security_challenge' => "Security Challenge:",
 	'row_security_code' => "Security Code:",
 	'text_slots' => "Slots:",
 	'text_unlimited' => "Unbegrenzt",