diff --git a/content/en/entities/Account.md b/content/en/entities/Account.md
index ff9b825075..bf4cc363c2 100644
--- a/content/en/entities/Account.md
+++ b/content/en/entities/Account.md
@@ -388,9 +388,10 @@ aliases: [
 #### `source[language]` {#source-language}
 
 **Description:** The default posting language for new statuses.\
-**Type:** String (ISO 639-1 language two-letter code) or empty string\
+**Type:** String (well-formed BCP 47 language tag, but parts other than language subtag may be discarded) or empty string\
 **Version history:**\
 2.4.2 - added
+4.1.0 - accept BCP 47
 
 #### `source[follow_requests_count]` {#follow_requests_count}
 
diff --git a/content/en/entities/Admin_Account.md b/content/en/entities/Admin_Account.md
index a70293be53..50f365525d 100644
--- a/content/en/entities/Admin_Account.md
+++ b/content/en/entities/Admin_Account.md
@@ -131,9 +131,10 @@ aliases: [
 ### `locale` {#locale}
 
 **Description:** The locale of the account.\
-**Type:** String (ISO 639 Part 1 two-letter language code)\
+**Type:** String (well-formed BCP 47 language tag, but parts other than language subtag may be discarded)\
 **Version history:**\
 2.9.1 - added
+4.1.0 - accept BCP 47
 
 ### `invite_request` {#invite_request}
 
diff --git a/content/en/entities/Instance.md b/content/en/entities/Instance.md
index 18a9e23eb2..cb57578b44 100644
--- a/content/en/entities/Instance.md
+++ b/content/en/entities/Instance.md
@@ -265,9 +265,10 @@ aliases: [
 ### `languages` {#languages}
 
 **Description:** Primary languages of the website and its staff.\
-**Type:** Array of String (ISO 639-1 two-letter code)\
+**Type:** Array of String (well-formed BCP 47 language tag, but parts other than language subtag may be discarded)\
 **Version history:**\
 4.0.0 - added
+4.1.0 - accept BCP 47
 
 ### `configuration` {#configuration}
 
diff --git a/content/en/entities/Preferences.md b/content/en/entities/Preferences.md
index 1fe69ae4d8..87c630d364 100644
--- a/content/en/entities/Preferences.md
+++ b/content/en/entities/Preferences.md
@@ -47,9 +47,10 @@ aliases: [
 ### `posting:default:language` {#posting-default-language}
 
 **Description:** Default language for new posts. Equivalent to [CredentialAccount#source\[language\]]({{< relref "entities/Account#source-language" >}})\
-**Type:** {{<nullable>}} String (ISO 639-1 language two-letter code), or null\
+**Type:** {{<nullable>}} String (well-formed BCP 47 language tag, but parts other than language subtag may be discarded), or null\
 **Version history:**\
 2.8.0 - added
+4.1.0 - accept BCP 47
 
 ### `reading:expand:media` {#reading-expand-media}
 
diff --git a/content/en/entities/Relationship.md b/content/en/entities/Relationship.md
index d513e5ed5b..640969b10e 100644
--- a/content/en/entities/Relationship.md
+++ b/content/en/entities/Relationship.md
@@ -65,9 +65,10 @@ aliases: [
 ### `languages` {#languages}
 
 **Description:** Which languages are you following from this user?\
-**Type:** Array of String (ISO 639-1 language two-letter code)\
+**Type:** Array of String (well-formed BCP 47 language tag, but parts other than language subtag may be discarded)\
 **Version history:**\
 4.0.0 - added
+4.1.0 - accept BCP 47
 
 ### `followed_by` {#followed_by}
 
diff --git a/content/en/entities/ScheduledStatus.md b/content/en/entities/ScheduledStatus.md
index 93171ec3a8..0d5459b9e7 100644
--- a/content/en/entities/ScheduledStatus.md
+++ b/content/en/entities/ScheduledStatus.md
@@ -169,9 +169,10 @@ Returned from `GET /api/v1/scheduled_statuses`:
 #### `params[language]` {#params-language}
 
 **Description:** The language that will be used for the status.\
-**Type:** {{<nullable>}} String (ISO 639-1 two-letter language code)\
+**Type:** {{<nullable>}} String (well-formed BCP 47 language tag, but parts other than language subtag may be discarded)\
 **Version history:**\
 2.7.0 - added
+4.1.0 - accept BCP 47
 
 #### `params[application_id]` {#params-application_id}
 
diff --git a/content/en/entities/Status.md b/content/en/entities/Status.md
index c603e209ff..a971636dcc 100644
--- a/content/en/entities/Status.md
+++ b/content/en/entities/Status.md
@@ -278,9 +278,10 @@ aliases: [
 ### `language` {#language}
 
 **Description:** Primary language of this status.\
-**Type:** {{<nullable>}} String (ISO 639 Part 1 two-letter language code) or null\
+**Type:** {{<nullable>}} String (well-formed BCP 47 language tag, but parts other than language subtag may be discarded) or null\
 **Version history:**\
 1.4.0 - added
+4.1.0 - accept BCP 47
 
 ### `text` {#text}
 
diff --git a/content/en/entities/Translation.md b/content/en/entities/Translation.md
index 67d0c475ec..8d0a86b7ca 100644
--- a/content/en/entities/Translation.md
+++ b/content/en/entities/Translation.md
@@ -32,9 +32,10 @@ aliases: [
 ### `detected_source_language` {#detected_source_language}
 
 **Description:** The language of the source text, as auto-detected by the machine translation provider.\
-**Type:** String (ISO 639 language code)\
+**Type:** String (well-formed BCP 47 language tag, but parts other than language subtag may be discarded)\
 **Version history:**\
 4.0.0 - added
+4.1.0 - accept BCP 47
 
 ### `provider` {#provider}
 
diff --git a/content/en/entities/V1_Instance.md b/content/en/entities/V1_Instance.md
index 276afc858d..248812b61a 100644
--- a/content/en/entities/V1_Instance.md
+++ b/content/en/entities/V1_Instance.md
@@ -240,9 +240,10 @@ aliases: [
 ### `languages` {#languages}
 
 **Description:** Primary languages of the website and its staff.\
-**Type:** Array of String (ISO 639-1 two-letter code)\
+**Type:** Array of String (well-formed BCP 47 language tag, but parts other than language subtag may be discarded)\
 **Version history:**\
 2.3.0 - added
+4.1.0 - accept BCP 47
 
 ### `registrations` {#registrations}
 
diff --git a/content/en/methods/accounts.md b/content/en/methods/accounts.md
index 5803a24928..e1518e2a0d 100644
--- a/content/en/methods/accounts.md
+++ b/content/en/methods/accounts.md
@@ -361,7 +361,7 @@ source[sensitive]
 : Boolean. Whether to mark authored statuses as sensitive by default.
 
 source[language]
-: String. Default language to use for authored statuses (ISO 6391)
+: String. Default language to use for authored statuses (in well-formed BCP 47 language tag, but parts other than language subtag may be discarded by the system).
 
 #### Response
 
@@ -1200,7 +1200,7 @@ notify
 : Boolean. Receive notifications when this account posts a status? Defaults to false.
 
 languages
-: Array of String (ISO 639-1 language two-letter code). Filter received statuses for these languages. If not provided, you will receive this account's posts in all languages.
+: Array of String (well-formed BCP 47 language tag, but parts other than language subtag may be discarded). Filter received statuses for these languages. If not provided, you will receive this account's posts in all languages.
 
 #### Response
 ##### 200: OK
diff --git a/content/en/methods/oauth.md b/content/en/methods/oauth.md
index a2714847b6..b4da968ae4 100644
--- a/content/en/methods/oauth.md
+++ b/content/en/methods/oauth.md
@@ -52,7 +52,7 @@ force_login
 : Boolean. Forces the user to re-login, which is necessary for authorizing with multiple accounts from the same instance.
 
 lang
-: String. The ISO 639-1 two-letter language code to use while rendering the authorization form.
+: String. The well-formed BCP 47 language tag (parts other than language subtag may be discarded by the system) to use while rendering the authorization form.
 
 #### Response
 ##### 200: OK
diff --git a/content/en/methods/statuses.md b/content/en/methods/statuses.md
index 87c59bafb9..28f1e8b037 100644
--- a/content/en/methods/statuses.md
+++ b/content/en/methods/statuses.md
@@ -75,7 +75,7 @@ visibility
 : String. Sets the visibility of the posted status to `public`, `unlisted`, `private`, `direct`.
 
 language
-: String. ISO 639 language code for this status.
+: String. Well-formed BCP 47 language tag for this status (parts other than language subtag may be discarded by the system).
 
 scheduled_at
 : String. ISO 8601 Datetime at which to schedule a status. Providing this parameter will cause ScheduledStatus to be returned instead of Status. Must be at least 5 minutes in the future.
@@ -546,7 +546,7 @@ Translate the status content into some language.
 ##### Form data parameters
 
 lang
-: String (ISO 639 language code). The status content will be translated into this language. Defaults to the user's current locale.
+: String (well-formed BCP 47 language tag, but parts other than language subtag may be discarded). The status content will be translated into this language. Defaults to the user's current locale.
 
 ##### Headers
 
@@ -1478,7 +1478,7 @@ sensitive
 : Boolean. Whether the status should be marked as sensitive.
 
 language
-: String. ISO 639 language code for the status.
+: String. Well-formed BCP 47 language tag for the status (parts other than language subtag may be discarded by the system).
 
 media_ids[]
 : Array of String. Include Attachment IDs to be attached as media. If provided, `status` becomes optional, and `poll` cannot be used.