diff --git a/modules/restapi/src/main/resources/docspell-openapi.yml b/modules/restapi/src/main/resources/docspell-openapi.yml
index dda2bce4f0..b149fae707 100644
--- a/modules/restapi/src/main/resources/docspell-openapi.yml
+++ b/modules/restapi/src/main/resources/docspell-openapi.yml
@@ -7266,8 +7266,10 @@ components:
           format: glob
         language:
           description: |
-            The language used for text extraction and analysis when
-            processing mails.
+            The language for text extraction and analysis when
+            processing mails. Use **ISO 639-3** (3-letter codes) such
+            as `eng`, `deu`, `fra`; ISO 639-1 (2-letter) like `en`,
+            `de` and language names like `english` are also accepted.
           type: string
           format: language
         postHandleAll:
@@ -8242,8 +8244,18 @@ components:
           type: string
           format: language
           description: |
-            The `language` of the document may be specified, otherwise
-            the one from settings is used.
+            The language of the document for processing (OCR, text
+            extraction, analysis). If not specified, the collective's
+            default language is used.
+
+            Use **ISO 639-3** (3-letter codes) such as `eng`, `deu`,
+            `fra`, `spa`. ISO 639-1 (2-letter) codes like `en`, `de`
+            and language names like `english`, `german` are also
+            accepted. Supported languages include German, English,
+            French, Italian, Spanish, Portuguese, Czech, Danish,
+            Finnish, Norwegian, Swedish, Russian, Romanian, Dutch,
+            Latvian, Japanese, Hebrew, Hungarian, Lithuanian, Polish,
+            Estonian, Ukrainian, Khmer, Slovak.
         attachmentsOnly:
           type: boolean
           default: false
@@ -8292,6 +8304,11 @@ components:
         language:
           type: string
           format: language
+          description: |
+            Default document language for the collective. Use **ISO
+            639-3** (3-letter codes) like `eng`, `deu`, `fra`; ISO
+            639-1 (2-letter) like `en`, `de` and language names are
+            also accepted.
         integrationEnabled:
           type: boolean
           description: |
@@ -8401,6 +8418,10 @@ components:
         language:
           type: string
           format: language
+          description: |
+            Document language for uploads via this source. Use **ISO
+            639-3** (3-letter codes) like `eng`, `deu`, `fra`; ISO
+            639-1 (2-letter) and language names are also accepted.
         created:
           description: DateTime
           type: integer
diff --git a/website/site/content/docs/api/upload.md b/website/site/content/docs/api/upload.md
index b02bcca74a..dbebe318e5 100644
--- a/website/site/content/docs/api/upload.md
+++ b/website/site/content/docs/api/upload.md
@@ -32,7 +32,7 @@ For example, here is a curl command uploading two files with meta
 data. Since `multiple` is `false`, both files are added to one item:
 
 ``` bash
-curl -XPOST -F meta='{"multiple":false, "direction": "outgoing", "tags": {"items":["Order"]}}' \
+curl -XPOST -F meta='{"multiple":false, "direction": "outgoing", "language": "eng", "tags": {"items":["Order"]}}' \
             -F file=@letter-en.pdf \
             -F file=@letter-de.pdf \
             http://192.168.1.95:7880/api/v1/open/upload/item/3H7hvJcDJuk-NrAW4zxsdfj-K6TMPyb6BGP-xKptVxUdqWa
@@ -89,9 +89,11 @@ specified via a JSON structure in a part with name `meta`:
   files or `*.html|*.pdf` for selecting html and pdf files. This only
   applies to archive files, like zip or e-mails (where this is applied
   to the attachments).
-- The `language` is used for processing the document(s) contained in
-  the request. If not specified the collective's default language is
-  used.
+- The `language` specifies the document language for processing (OCR,
+  text extraction, analysis). If not specified, the collective's
+  default language is used. Use **ISO 639-3** (3-letter codes) such as
+  `eng`, `deu`, `fra`, `spa`. ISO 639-1 (2-letter) codes like `en`,
+  `de` and language names like `english`, `german` are also accepted.
 - The `attachmentsOnly` property only applies to e-mail files (usually
   `*.eml`). If this is `true`, then the e-mail body is discarded and
   only the attachments are imported. An e-mail without any attachments
diff --git a/website/site/content/docs/webapp/metadata.md b/website/site/content/docs/webapp/metadata.md
index 2fd2929ebf..a36a5c8230 100644
--- a/website/site/content/docs/webapp/metadata.md
+++ b/website/site/content/docs/webapp/metadata.md
@@ -131,11 +131,12 @@ page](@/docs/webapp/customfields.md) for more information.
 
 An important setting is the language of your documents. This helps OCR
 and text analysis. You can select between various languages. The
-language can also specified with each [upload
-request](@/docs/api/upload.md).
+language can also be specified with each [upload
+request](@/docs/api/upload.md) using ISO 639-3 codes (e.g. `eng`,
+`deu`) or ISO 639-1 (e.g. `en`, `de`).
 
 Go to the *Collective Settings* page and click *Document
-Language*. This will set the lanugage for all your documents.
+Language*. This will set the language for all your documents.
 
 The language has effects in several areas: text extraction, fulltext
 search and text analysis. When extracting text from images, tesseract