Update localization docs: BCP-47 codes and fallback behavior (#5202)

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Co-authored-by: bot_apk <apk@cognition.ai>
Co-authored-by: Devin Logan <devinannlogan@gmail.com>

Author: 4 people

.vale/styles/FernStyles/Acronyms.yml

@@ -106,3 +106,4 @@ exceptions:
   - WSS
   - GEO
   - SPDX
+  - BCP

.vale/styles/FernStyles/Headings.yml

@@ -88,3 +88,4 @@ exceptions:
   - WSS
   - GEO
   - OG
+  - BCP

fern/products/docs/pages/localization/overview.mdx

@@ -33,7 +33,7 @@ The manual setup below works today. Once localization is generally available, mo
 
 <Step title="Add a translations folder">
 
-Create a `translations` folder inside your `fern` directory, with a subfolder for each language using its [ISO 639-1 code](https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes) (e.g., `fr`, `ja`). Mirror your `pages/` structure inside each language folder.
+Create a `translations` folder inside your `fern` directory, with a subfolder for each language using its [BCP 47 language tag](https://en.wikipedia.org/wiki/IETF_language_tag) (e.g., `fr-FR`, `ja-JP`). Mirror your `pages/` structure inside each language folder.
 
 <Files>
   <Folder name="fern" defaultOpen>
@@ -44,13 +44,13 @@ Create a `translations` folder inside your `fern` directory, with a subfolder fo
       <File name="getting-started.mdx" />
     </Folder>
     <Folder name="translations" defaultOpen highlighted>
-      <Folder name="fr" defaultOpen>
+      <Folder name="fr-FR" defaultOpen>
         <Folder name="pages" defaultOpen>
           <File name="introduction.mdx" />
           <File name="getting-started.mdx" />
         </Folder>
       </Folder>
-      <Folder name="ja" defaultOpen>
+      <Folder name="ja-JP" defaultOpen>
         <Folder name="pages" defaultOpen>
           <File name="introduction.mdx" />
           <File name="getting-started.mdx" />
@@ -71,9 +71,9 @@ instances:
   - url: your-org.docs.buildwithfern.com
 
 translations:
-  - lang: en
-  - lang: fr
-  - lang: ja
+  - lang: en-US
+  - lang: fr-FR
+  - lang: ja-JP
 ```
 
 </Step>
@@ -82,14 +82,18 @@ translations:
 
 Each translated `.mdx` mirrors its source page's content. Use the `sidebar-title` frontmatter field to override the sidebar entry per language:
 
-```mdx translations/fr/pages/introduction.mdx
+```mdx translations/fr-FR/pages/introduction.mdx
 ---
 sidebar-title: Introduction
 ---
 
 Bienvenue dans la documentation.
 ```
 
+<Tip>
+  You only need to translate the files you want to localize. Anything missing from a language folder — pages, images, or other assets — falls back to the default-language version.
+</Tip>
+
 </Step>
 
 <Step title="Generate your docs">