diff --git a/src/i18n/locales.keys.test.ts b/src/i18n/locales.keys.test.ts
new file mode 100644
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+++ b/src/i18n/locales.keys.test.ts
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+// Guard: fr.json and en.json declare the same keys (#453).
+//
+// i18next serves fr.json, the default language, and en.json, the fallback
+// (`src/i18n/config.ts`). A key written in fr.json only shows raw to an
+// English user; one written in en.json only shows in English to a French
+// user. Neither breaks the build. These tests hold both files to one set of
+// keys.
+//
+// A key is the dotted path of a leaf, array indices included
+// (`docs.dashboard.features.6`) and plural suffixes too
+// (`import.sources.fileCount_one`). Values are never compared: a text left
+// untranslated in the other file passes. A path that is an array in one file
+// and an object in the other, or a leaf in one and an object or an array in
+// the other, is a gap as well: `t(key, { returnObjects: true })` would hand
+// the component another shape.
+
+import { describe, it, expect } from "vitest";
+import fr from "./locales/fr.json";
+import en from "./locales/en.json";
+
+type NodeKind = "object" | "array" | "leaf";
+
+/** Every node under the root of a locale tree, by dotted path (array indices included), with its kind. */
+function nodesOf(tree: unknown): Map<string, NodeKind> {
+  const nodes = new Map<string, NodeKind>();
+  const visit = (value: unknown, path: string[]): void => {
+    const kind: NodeKind = Array.isArray(value)
+      ? "array"
+      : typeof value === "object" && value !== null
+        ? "object"
+        : "leaf";
+    if (path.length > 0) nodes.set(path.join("."), kind);
+    if (kind === "leaf") return;
+    // An array's entries are its indices, as strings: `features.6`.
+    for (const [key, item] of Object.entries(value as object)) visit(item, [...path, key]);
+  };
+  visit(tree, []);
+  return nodes;
+}
+
+/**
+ * The keys of `from` that `into` lacks, sorted: the leaves of `from` with
+ * nothing at the same path in `into`. A path `into` holds with another kind is
+ * left to `kindGaps`.
+ */
+function missingKeys(from: Map<string, NodeKind>, into: Map<string, NodeKind>): string[] {
+  return [...from]
+    .filter(([path, kind]) => kind === "leaf" && !into.has(path))
+    .map(([path]) => path)
+    .sort();
+}
+
+/** The paths both trees hold with different kinds, sorted, as `path: <kind> in <a>, <kind> in <b>`. */
+function kindGaps(
+  a: Map<string, NodeKind>,
+  b: Map<string, NodeKind>,
+  nameA: string,
+  nameB: string,
+): string[] {
+  return [...a]
+    .filter(([path, kind]) => b.has(path) && b.get(path) !== kind)
+    .map(([path, kind]) => `${path}: ${kind} in ${nameA}, ${b.get(path)} in ${nameB}`)
+    .sort();
+}
+
+const FR = nodesOf(fr);
+const EN = nodesOf(en);
+
+describe("locale keys: fr.json and en.json (#453)", () => {
+  it("reads both files whole, array items included", () => {
+    // The canary: an empty or unread file would make the comparison pass on nothing.
+    for (const [name, nodes] of [["fr.json", FR], ["en.json", EN]] as const) {
+      expect(nodes.get("common.cancel"), name).toBe("leaf");
+      expect(nodes.get("dashboard.help.tips"), name).toBe("array");
+      expect(nodes.get("dashboard.help.tips.0"), name).toBe("leaf");
+    }
+  });
+
+  it("en.json declares every key of fr.json", () => {
+    expect(missingKeys(FR, EN), "keys of fr.json missing from en.json").toEqual([]);
+  });
+
+  it("fr.json declares every key of en.json", () => {
+    expect(missingKeys(EN, FR), "keys of en.json missing from fr.json").toEqual([]);
+  });
+
+  it("gives every path both files hold the same kind: object, array or leaf", () => {
+    expect(kindGaps(FR, EN, "fr.json", "en.json"), "paths of another kind").toEqual([]);
+  });
+});
+
+describe("locale keys: the comparison (#453)", () => {
+  it("names, on each side apart, the keys the other side lacks, array indices included", () => {
+    const a = nodesOf({ list: ["one", "two"], only: { a: "x" } });
+    const b = nodesOf({ list: ["one"], only: { b: "y" } });
+    expect(missingKeys(a, b)).toEqual(["list.1", "only.a"]);
+    expect(missingKeys(b, a)).toEqual(["only.b"]);
+  });
+
+  it("holds an array against an object with the same indices as a gap", () => {
+    const a = nodesOf({ list: ["one"] });
+    const b = nodesOf({ list: { 0: "one" } });
+    expect(missingKeys(a, b)).toEqual([]);
+    expect(missingKeys(b, a)).toEqual([]);
+    expect(kindGaps(a, b, "a", "b")).toEqual(["list: array in a, object in b"]);
+  });
+
+  it("holds a leaf against an object or an array as a gap", () => {
+    const leaf = nodesOf({ x: "text" });
+    expect(kindGaps(leaf, nodesOf({ x: { y: "text" } }), "a", "b")).toEqual([
+      "x: leaf in a, object in b",
+    ]);
+    expect(kindGaps(leaf, nodesOf({ x: ["text"] }), "a", "b")).toEqual(["x: leaf in a, array in b"]);
+    // The leaf is not reported missing as well: `x` exists on the other side, with another kind.
+    expect(missingKeys(leaf, nodesOf({ x: ["text"] }))).toEqual([]);
+  });
+
+  it("never compares values", () => {
+    const a = nodesOf({ x: "un", list: ["a", "b"] });
+    const b = nodesOf({ x: "one", list: ["c", "d"] });
+    expect(missingKeys(a, b)).toEqual([]);
+    expect(missingKeys(b, a)).toEqual([]);
+    expect(kindGaps(a, b, "a", "b")).toEqual([]);
+  });
+});