feat: exposes locale within preview function

docs/configuration/collections.mdx

@@ -70,7 +70,10 @@ Collection `admin` options can accept a `preview` function that will be used to
 
 If the function is specified, a Preview button will automatically appear in the corresponding collection's Edit view. Clicking the Preview button will link to the URL that is generated by the function.
 
-The preview function accepts the document being edited as an argument.
+**The preview function accepts two arguments:**
+
+1. The document being edited
+1. An `options` object, containing `locale` and `token` properties. The `token` is the currently logged in user's JWT.
 
 **Example collection with preview function:**
 
@@ -85,9 +88,9 @@ The preview function accepts the document being edited as an argument.
 		}
 	]
 	admin: {
-		preview: (doc) => {
+		preview: (doc, { locale }) => {
 			if (doc?.slug) {
-				return `https://bigbird.com/preview/posts/${doc.slug}`,
+				return `https://bigbird.com/preview/posts/${doc.slug}?locale=${locale}`,
 			}
 
 			return null;

src/admin/components/elements/PreviewButton/index.tsx

@@ -2,14 +2,16 @@ import React from 'react';
 import { useAuth } from '@payloadcms/config-provider';
 import Button from '../Button';
 import { Props } from './types';
+import { useLocale } from '../../utilities/Locale';
 
 const baseClass = 'preview-btn';
 
 const PreviewButton: React.FC<Props> = ({ generatePreviewURL, data }) => {
+  const locale = useLocale();
   const { token } = useAuth();
 
   if (generatePreviewURL && typeof generatePreviewURL === 'function') {
-    const url = generatePreviewURL(data, token);
+    const url = generatePreviewURL(data, { locale, token });
 
     return (
       <Button

src/admin/components/elements/PreviewButton/types.ts

@@ -1,6 +1,7 @@
 import { Data } from '../../forms/Form/types';
+import { GeneratePreviewURL } from '../../../../collections/config/types';
 
 export type Props = {
-  generatePreviewURL?: (data: unknown, token: string) => string,
+  generatePreviewURL?: GeneratePreviewURL,
   data?: Data
 }

src/collections/config/types.ts

@@ -90,6 +90,13 @@ export type AfterForgotPasswordHook = (args?: {
   args?: any;
 }) => any;
 
+type GeneratePreviewURLOptions = {
+  locale: string
+  token: string
+}
+
+export type GeneratePreviewURL = (doc: Document, options: GeneratePreviewURLOptions) => string
+
 export type PayloadCollectionConfig = {
   slug: string;
   labels?: {
@@ -108,7 +115,7 @@ export type PayloadCollectionConfig = {
       }
     };
     enableRichTextRelationship?: boolean
-    preview?: (doc: Document, token: string) => string
+    preview?: GeneratePreviewURL
   };
   hooks?: {
     beforeOperation?: BeforeOperationHook[];