fix: use req.t for the ValidationError message when t is not passed

ValidationError only translated its "The following field is invalid"
prefix when a t function was passed as the second argument, otherwise
it used the English strings. Errors thrown from hooks with just req
were always shown in English in the admin toast.

The constructor now falls back to req.t when t is not passed, before
using the English strings.

packages/payload/src/errors/ValidationError.spec.ts

@@ -0,0 +1,22 @@
+import { initI18n } from '@payloadcms/translations'
+import { de } from '@payloadcms/translations/languages/de'
+import { describe, expect, it } from 'vitest'
+
+import { ValidationError } from './ValidationError.js'
+
+describe('ValidationError', () => {
+  it('should translate the message with req.t when no t is passed', async () => {
+    const i18n = await initI18n({
+      config: { fallbackLanguage: 'de', supportedLanguages: { de } },
+      context: 'api',
+      language: 'de',
+    })
+
+    const error = new ValidationError({
+      errors: [{ message: 'Invalides Passwort', path: 'password' }],
+      req: { i18n, t: i18n.t },
+    })
+
+    expect(error.message).toBe('Das folgende Feld ist nicht korrekt: password')
+  })
+})

packages/payload/src/errors/ValidationError.ts

@@ -37,13 +37,15 @@ export class ValidationError extends APIError<{
       id?: number | string
       /**
        *  req needs to be passed through (if you have one) in order to resolve label functions that may be part of the errors array
+       *  and to translate the message when `t` is not passed
        */
       req?: Partial<PayloadRequest>
     },
     t?: TFunction,
   ) {
-    const message = t
-      ? t('error:followingFieldsInvalid', { count: results.errors.length })
+    const translate = t ?? results.req?.t
+    const message = translate
+      ? translate('error:followingFieldsInvalid', { count: results.errors.length })
       : results.errors.length === 1
         ? en.translations.error.followingFieldsInvalid_one
         : en.translations.error.followingFieldsInvalid_other