diff --git a/crates/engine/src/cmd/format.rs b/crates/engine/src/cmd/format.rs
index c68d1242..1b9aee83 100644
--- a/crates/engine/src/cmd/format.rs
+++ b/crates/engine/src/cmd/format.rs
@@ -59,6 +59,15 @@ pub fn specs() -> Vec<CommandSpec> {
             case
         ),
         cmd!("format.caps", "Caps", [], None, "{caps: none|small|all}", has_text_or_shapes, caps),
+        cmd!(
+            "format.language",
+            "Language",
+            ["Review", "Language"],
+            None,
+            "{lang: BCP 47 tag (en-US, fr-FR…)} — the proofing language of the selected text",
+            has_text_or_shapes,
+            language
+        ),
         cmd!(
             "format.spacing",
             "Character Spacing",
@@ -297,6 +306,14 @@ fn highlight(s: &mut Session, p: &Value) -> Result<Value> {
     let c = color_param(p, "color");
     format_run(s, &move |r| r.highlight = c.clone())
 }
+fn language(s: &mut Session, p: &Value) -> Result<Value> {
+    let l = str_param(p, "lang")
+        .map(str::trim)
+        .filter(|l| !l.is_empty() && l.len() <= 35)
+        .ok_or_else(|| bad("format.language", "missing or invalid `lang`"))?
+        .to_string();
+    format_run(s, &move |r| r.lang = Some(l.clone()))
+}
 fn caps(s: &mut Session, p: &Value) -> Result<Value> {
     let c = match str_param(p, "caps").unwrap_or("none") {
         "small" => Caps::Small,
@@ -678,6 +695,7 @@ fn state(s: &mut Session, _p: &Value) -> Result<Value> {
         "align": pp.align.map(|a| a.xml()),
         "bullets": matches!(pp.bullet, Some(Bullet::Char { .. })),
         "numbering": matches!(pp.bullet, Some(Bullet::AutoNum { .. })),
+        "lang": r.lang,
         "lineSpacing": serde_json::to_value(pp.line_spacing).unwrap_or_default(),
         "painter": s.painter.is_some(),
     }))
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index e83f72ef..ad87d48f 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -835,3 +835,17 @@ fn slide_ranges_parse_like_powerpoints_print_box() {
     let opts = cmd::file::pdf_options(&json!({"slides": [0, 7, 1]}), 2).unwrap();
     assert_eq!(opts.slides, Some(vec![0, 1]), "indices past the end are dropped");
 }
+
+#[test]
+fn format_language_marks_the_selected_text() {
+    let mut s = session();
+    let title = s.doc().unwrap().current_slide().unwrap().shapes[0].id.0;
+    s.execute("text.edit", &json!({"id": title})).unwrap();
+    s.execute("text.insert", &json!({"text": "Hola mundo"})).unwrap();
+    s.execute("text.move", &json!({"to": "wordLeft", "extend": true})).unwrap();
+    s.execute("format.language", &json!({"lang": "es-ES"})).unwrap();
+    assert_eq!(s.execute("format.state", &json!({})).unwrap()["lang"], "es-ES");
+    let body = s.doc().unwrap().current_slide().unwrap().shapes[0].text.clone().unwrap();
+    let langs: Vec<_> = body.paragraphs[0].runs.iter().map(|r| (r.text.as_str(), r.props.lang.as_deref())).collect();
+    assert_eq!(langs, vec![("Hola ", None), ("mundo", Some("es-ES"))]);
+}
diff --git a/crates/ui-egui/src/dialogs.rs b/crates/ui-egui/src/dialogs.rs
index d1934be9..74ec0c91 100644
--- a/crates/ui-egui/src/dialogs.rs
+++ b/crates/ui-egui/src/dialogs.rs
@@ -556,21 +556,32 @@ fn body(app: &mut SlideApp, ui: &mut Ui, d: &mut Dialog) -> bool {
         }
         "language" => {
             ui.label("Mark selected text as:");
-            for l in [
-                "English (United States)",
-                "English (United Kingdom)",
-                "Español",
-                "Français",
-                "Deutsch",
-                "Italiano",
-                "Português",
-                "日本語",
-                "中文",
-                "العربية",
-            ] {
-                let _ = ui.selectable_label(l.starts_with("English (United States)"), l);
+            if !d.fields.contains_key("lang") {
+                let cur = app.session.execute("format.state", &json!({})).ok().and_then(|v| v.get("lang").and_then(Value::as_str).map(String::from));
+                d.fields.insert("lang".into(), cur.unwrap_or_else(|| "en-US".into()));
             }
+            let mut lang = d.get("lang", "en-US");
+            for (tag, name) in [
+                ("en-US", "English (United States)"),
+                ("en-GB", "English (United Kingdom)"),
+                ("es-ES", "Español"),
+                ("fr-FR", "Français"),
+                ("de-DE", "Deutsch"),
+                ("it-IT", "Italiano"),
+                ("pt-PT", "Português"),
+                ("ja-JP", "日本語"),
+                ("zh-CN", "中文"),
+                ("ar-SA", "العربية"),
+            ] {
+                if ui.selectable_label(lang.eq_ignore_ascii_case(tag), name).clicked() {
+                    lang = tag.to_string();
+                }
+            }
+            d.fields.insert("lang".into(), lang.clone());
             let (ok, cancel) = buttons(ui, "OK");
+            if ok {
+                run(app, "format.language", json!({"lang": lang}));
+            }
             ok || cancel
         }
         "customShow" => {
diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index ec97e6e6..19b7aec9 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -1408,4 +1408,33 @@ mod tests {
         harness.run_steps(3);
         assert_eq!(harness.state().textures.canvas_renders, n, "an unchanged slide isn't rendered again");
     }
+
+    /// #339: clicking a language in Review > Language did nothing, and OK didn't mark the text.
+    #[test]
+    fn the_language_dialog_marks_the_selected_text() {
+        use egui_kittest::kittest::Queryable;
+        let mut harness = egui_kittest::Harness::builder().with_size(egui::vec2(1200.0, 800.0)).build_ui_state(
+            |ui, app: &mut SlideApp| {
+                app.logic(ui.ctx());
+                app.ui(ui);
+            },
+            SlideApp::new(Session::with_new(), Services::default()),
+        );
+        harness.run_steps(2);
+        let title = harness.state().session.active().unwrap().doc.slides[0].shapes[0].id.0;
+        harness.state_mut().session.execute("text.edit", &json!({"id": title})).unwrap();
+        harness.state_mut().session.execute("text.insert", &json!({"text": "Bonjour"})).unwrap();
+        harness.state_mut().session.execute("text.selectAll", &json!({})).unwrap();
+        harness.state_mut().run("app.dialog", json!({"id": "language"})).unwrap();
+        // The anchored window settles over its first frames; click once it stays put.
+        harness.run_steps(4);
+        harness.get_by_label("Français").click();
+        harness.run_steps(2);
+        harness.get_by_label("OK").click();
+        harness.run_steps(2);
+        assert!(harness.state().dialog.is_none());
+        let d = harness.state().session.active().unwrap();
+        let body = d.doc.slides[0].shapes[0].text.as_ref().unwrap();
+        assert_eq!(body.paragraphs[0].runs[0].props.lang.as_deref(), Some("fr-FR"));
+    }
 }