diff --git a/resources/lang/en.json b/resources/lang/en.json
index 41e04b5d54..5425260b5d 100644
--- a/resources/lang/en.json
+++ b/resources/lang/en.json
@@ -1180,6 +1180,13 @@
     "teams_of": "{teamCount} teams of {playersPerTeam}",
     "teams_title": "Teams"
   },
+  "multi_tab": {
+    "detected": "Multiple open game tabs detected.",
+    "explanation": "To ensure fair play, only one active game tab is permitted at a time.",
+    "please_wait": "Please wait",
+    "seconds": "seconds",
+    "warning": "Vigilant Mode Warning"
+  },
   "new_lobby_prompt": {
     "confirm": "Start a new lobby? All players will be invited to join.",
     "dismiss": "Dismiss",
diff --git a/tests/client/MultiTabModal.test.ts b/tests/client/MultiTabModal.test.ts
new file mode 100644
index 0000000000..220972be6d
--- /dev/null
+++ b/tests/client/MultiTabModal.test.ts
@@ -0,0 +1,26 @@
+import fs from "fs";
+import path from "path";
+import { describe, expect, it } from "vitest";
+
+const EN_JSON = path.join(
+  __dirname,
+  "..",
+  "..",
+  "resources",
+  "lang",
+  "en.json",
+);
+
+describe("multi_tab translations", () => {
+  it("includes all multi_tab modal translation keys in en.json", () => {
+    const en = JSON.parse(fs.readFileSync(EN_JSON, "utf8"));
+    expect(en.multi_tab).toBeDefined();
+    expect(en.multi_tab.warning).toBe("Vigilant Mode Warning");
+    expect(en.multi_tab.detected).toBe("Multiple open game tabs detected.");
+    expect(en.multi_tab.please_wait).toBe("Please wait");
+    expect(en.multi_tab.seconds).toBe("seconds");
+    expect(en.multi_tab.explanation).toBe(
+      "To ensure fair play, only one active game tab is permitted at a time.",
+    );
+  });
+});