diff --git a/crates/ui-egui/src/control.rs b/crates/ui-egui/src/control.rs
index 2ea5cfae0..c8016f34b 100644
--- a/crates/ui-egui/src/control.rs
+++ b/crates/ui-egui/src/control.rs
@@ -175,6 +175,7 @@ pub fn handle(app: &mut SheetApp, ctx: &egui::Context, method: &str, p: &Json) -
         }
         "engine.commands" => {
             let mut v: Vec<Json> = app.session.commands().into_iter().map(|c| serde_json::to_value(c).unwrap_or_default()).collect();
+            let language_params = format!("{{language: {}}} (alias: code)", crate::i18n::Language::code_list("|"));
             for (id, label, params) in [
                 ("ui.ribbonTab", "Ribbon Tab", ""),
                 ("view.collapseRibbon", "Collapse Ribbon", ""),
@@ -195,7 +196,7 @@ pub fn handle(app: &mut SheetApp, ctx: &egui::Context, method: &str, p: &Json) -
                     "Submit Print Job",
                     "existing file.print sheet/pagination params → {printing,bytes,pending}; submits without opening a dialog",
                 ),
-                ("app.language.set", "Interface Language", "{language: \"en\"|\"zh\"|\"ja\"|\"ko\"|\"ru\"|\"pt\"|\"de\"|\"cs\"} (alias: code)"),
+                ("app.language.set", "Interface Language", language_params.as_str()),
                 ("app.language.english", "Interface Language: English", ""),
                 ("app.language.japanese", "Interface Language: Japanese", ""),
             ] {
@@ -530,4 +531,19 @@ mod tests {
         assert!(!app.printing.busy(), "discovery must not submit a job");
         assert!(app.backstage.is_none(), "discovery must not open settings");
     }
+
+    #[test]
+    fn language_command_docs_list_every_language() {
+        let mut session = gridcraft_engine::Session::new();
+        session.new_workbook();
+        let mut app = SheetApp::new(session, crate::Services::default());
+        let Outcome::Done(reply) = handle(&mut app, &egui::Context::default(), "engine.commands", &json!({"search":"language"})) else {
+            panic!("command discovery must return immediately");
+        };
+        let commands = reply["result"].as_array().unwrap();
+        let docs = commands.iter().find(|c| c["id"] == "app.language.set").unwrap()["params"].as_str().unwrap();
+        for l in crate::i18n::Language::ALL {
+            assert!(docs.contains(&format!("\"{}\"", l.code())), "app.language.set docs miss {:?}: {docs}", l.code());
+        }
+    }
 }
diff --git a/crates/ui-egui/src/i18n.rs b/crates/ui-egui/src/i18n.rs
index 44bc34d5b..be92fd779 100644
--- a/crates/ui-egui/src/i18n.rs
+++ b/crates/ui-egui/src/i18n.rs
@@ -78,6 +78,12 @@ impl Language {
         Self::from_tag(code)
     }
 
+    /// Every code `app.language.set` takes, quoted and joined (`"en"|"es"|…`), for command docs
+    /// and errors.
+    pub fn code_list(separator: &str) -> String {
+        Self::ALL.iter().map(|l| format!("\"{}\"", l.code())).collect::<Vec<_>>().join(separator)
+    }
+
     fn from_tag(tag: &str) -> Option<Self> {
         match tag.trim().split(['-', '_']).next()?.to_ascii_lowercase().as_str() {
             "en" => Some(Self::En),
@@ -774,6 +780,10 @@ mod tests {
         app.run("app.language.set", serde_json::json!({"code": "ja"})).unwrap();
         assert_eq!(app.ui.language, Language::Ja, "`code` is an alias for `language`");
         assert!(app.run("app.language.set", serde_json::json!({})).is_err(), "no language is an error, not a dialog");
+        let unknown = app.run("app.language.set", serde_json::json!({"language": "xx"})).unwrap_err().to_string();
+        for l in Language::ALL {
+            assert!(unknown.contains(&format!("\"{}\"", l.code())), "the error lists {:?}: {unknown}", l.code());
+        }
         app.run("app.language.english", serde_json::json!({})).unwrap();
         assert_eq!(app.ui.language, Language::En);
         for l in Language::ALL {
diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index 1b08f5d7f..86e543120 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -658,7 +658,7 @@ impl SheetApp {
                         self.ui.language = l;
                         Ok(json!({"language": l}))
                     }
-                    None => Err(format!("unknown language {code:?} (use \"en\", \"es\", \"zh\", \"ja\", \"ko\", \"ru\", \"pt\" or \"cs\")")),
+                    None => Err(format!("unknown language {code:?} (use one of {})", i18n::Language::code_list(", "))),
                 }
             }
             "app.language.english" => {
diff --git a/docs/control-protocol.md b/docs/control-protocol.md
index e82ae7299..af73ca515 100644
--- a/docs/control-protocol.md
+++ b/docs/control-protocol.md
@@ -56,8 +56,9 @@ the View ▸ Animations checkbox, saved with the other UI preferences and on by
 
 `{command: "app.language.set", params: {language: "ja"}}` switches the interface language
 (`code` is accepted as an alias for `language`); `app.language.english` and
-`app.language.japanese` are shortcuts. Only interface chrome is translated, never command ids or
-document text. The choice is saved in `ui.json`; with nothing saved the desktop app follows the
+`app.language.japanese` are shortcuts. `engine.commands` lists every code it takes (`en`, `es`,
+`zh`, `ja`, `ko`, `ru`, `pt`, `de`, `cs`, `fr`, `sr`), as does the error for an unknown one. Only
+interface chrome is translated, never command ids or document text. The choice is saved in `ui.json`; with nothing saved the desktop app follows the
 system language.
 
 `{command: "file.open", params: {path: "Secret.xlsx", password: "…"}}` opens a password-protected