diff --git a/crates/ruma-common/src/events/message.rs b/crates/ruma-common/src/events/message.rs
index d7562168..04a15c4a 100644
--- a/crates/ruma-common/src/events/message.rs
+++ b/crates/ruma-common/src/events/message.rs
@@ -265,10 +265,16 @@ pub struct TextRepresentation {
     ///
     /// This must be a valid language code according to [BCP 47](https://www.rfc-editor.org/rfc/bcp/bcp47.txt).
     ///
+    /// This is optional and defaults to `en`.
+    ///
     /// [MSC3554]: https://github.com/matrix-org/matrix-spec-proposals/pull/3554
     #[cfg(feature = "unstable-msc3554")]
-    #[serde(skip_serializing_if = "Option::is_none")]
-    pub lang: Option<String>,
+    #[serde(
+        rename = "org.matrix.msc3554.lang",
+        default = "Text::default_lang",
+        skip_serializing_if = "Text::is_default_lang"
+    )]
+    pub lang: String,
 }
 
 impl TextRepresentation {
@@ -278,7 +284,7 @@ impl TextRepresentation {
             mimetype: mimetype.into(),
             body: body.into(),
             #[cfg(feature = "unstable-msc3554")]
-            lang: None,
+            lang: Self::default_lang(),
         }
     }
 
@@ -309,4 +315,12 @@ impl TextRepresentation {
     fn is_default_mimetype(mime: &str) -> bool {
         mime == "text/plain"
     }
+
+    fn default_lang() -> String {
+        "en".to_owned()
+    }
+
+    fn is_default_lang(lang: &str) -> bool {
+        lang == "en"
+    }
 }
diff --git a/crates/ruma-common/tests/events/message.rs b/crates/ruma-common/tests/events/message.rs
index 1897c55b..86806fea 100644
--- a/crates/ruma-common/tests/events/message.rs
+++ b/crates/ruma-common/tests/events/message.rs
@@ -315,18 +315,18 @@ fn emote_event_deserialization() {
 #[cfg(feature = "unstable-msc3554")]
 fn lang_serialization() {
     let content = TextContentBlock::try_from(vec![
-        assign!(TextRepresentation::plain("Bonjour le monde !"), { lang: Some("fr".into()) }),
-        assign!(TextRepresentation::plain("Hallo Welt!"), { lang: Some("de".into()) }),
-        assign!(TextRepresentation::plain("Hello World!"), { lang: Some("en".into()) }),
+        assign!(TextRepresentation::plain("Bonjour le monde !"), { lang: "fr".into() }),
+        assign!(TextRepresentation::plain("Hallo Welt!"), { lang: "de".into() }),
+        assign!(TextRepresentation::plain("Hello World!"), { lang: "en".into() }),
     ])
     .unwrap();
 
     assert_eq!(
         to_json_value(content).unwrap(),
         json!([
-            { "body": "Bonjour le monde !", "lang": "fr"},
-            { "body": "Hallo Welt!", "lang": "de"},
-            { "body": "Hello World!", "lang": "en"},
+            { "body": "Bonjour le monde !", "org.matrix.msc3554.lang": "fr"},
+            { "body": "Hallo Welt!", "org.matrix.msc3554.lang": "de"},
+            { "body": "Hello World!"},
         ])
     );
 }
@@ -335,15 +335,15 @@ fn lang_serialization() {
 #[cfg(feature = "unstable-msc3554")]
 fn lang_deserialization() {
     let json_data = json!([
-        { "body": "Bonjour le monde !", "lang": "fr"},
-        { "body": "Hallo Welt!", "lang": "de"},
-        { "body": "Hello World!", "lang": "en"},
+        { "body": "Bonjour le monde !", "org.matrix.msc3554.lang": "fr"},
+        { "body": "Hallo Welt!", "org.matrix.msc3554.lang": "de"},
+        { "body": "Hello World!"},
     ]);
 
     let content = from_json_value::<TextContentBlock>(json_data).unwrap();
-    assert_eq!(content[0].lang.as_deref(), Some("fr"));
-    assert_eq!(content[1].lang.as_deref(), Some("de"));
-    assert_eq!(content[2].lang.as_deref(), Some("en"));
+    assert_eq!(content[0].lang, "fr");
+    assert_eq!(content[1].lang, "de");
+    assert_eq!(content[2].lang, "en");
 }
 
 #[test]