fix(i18n): count plural forms only large round numbers use

frontend/scripts/i18n-check.mjs

@@ -1,6 +1,7 @@
 // Lists each locale's keys missing from, or no longer in, en.json: `npm run i18n:check [code]`.
 // A plural key (key_one, key_other, ...) needs every form the locale uses for whole-number counts.
 import { readdirSync, readFileSync } from "node:fs";
+import { pluralForms } from "./plural-forms.mjs";
 
 const dir = new URL("../src/i18n/locales/", import.meta.url);
 const PLURAL = /_(zero|one|two|few|many|other)$/;
@@ -13,12 +14,6 @@ function keys(node, prefix = "") {
 
 const read = (code) => keys(JSON.parse(readFileSync(new URL(`${code}.json`, dir), "utf8")));
 
-// Fractional-only forms (Lithuanian `many`) are left out: counts here are whole numbers.
-function pluralForms(code) {
-  const rules = new Intl.PluralRules(code);
-  return new Set(Array.from({ length: 1000 }, (_, n) => rules.select(n)));
-}
-
 const plurals = new Set();
 const plain = [];
 for (const key of read("en")) {
@@ -40,7 +35,7 @@ if (only && codes.length === 0) {
 for (const code of codes) {
   const forms = [...pluralForms(code)];
   const en = new Set([...plain, ...[...plurals].flatMap((base) => forms.map((form) => `${base}_${form}`))]);
-  // Extra plural forms a translator added (such as `_many`) aren't stale, just unused for whole numbers.
+  // Extra plural forms a translator added (Lithuanian `_many`) aren't stale, just unused by this locale's whole numbers.
   const locale = new Set(read(code).filter((key) => en.has(key) || !plurals.has(key.replace(PLURAL, ""))));
   const missing = [...en].filter((key) => !locale.has(key));
   const stale = [...locale].filter((key) => !en.has(key));

frontend/scripts/plural-forms.mjs

@@ -0,0 +1,9 @@
+// The plural forms a locale uses for whole-number counts: 0-999 plus powers of ten, so forms only large
+// round numbers take (French and Spanish `many` for 1,000,000) count too. Fractional-only forms (Lithuanian
+// `many`) are left out.
+const SAMPLES = [...Array.from({ length: 1000 }, (_, n) => n), ...Array.from({ length: 10 }, (_, k) => 10 ** (k + 3))];
+
+export function pluralForms(code) {
+  const rules = new Intl.PluralRules(code);
+  return new Set(SAMPLES.map((n) => rules.select(n)));
+}

frontend/scripts/plural-forms.test.mjs

@@ -0,0 +1,14 @@
+import { describe, expect, it } from "vitest";
+import { pluralForms } from "./plural-forms.mjs";
+
+describe("pluralForms", () => {
+  it("includes forms only large round counts use", () => {
+    expect([...pluralForms("fr")].toSorted()).toEqual(["many", "one", "other"]);
+    expect([...pluralForms("es")].toSorted()).toEqual(["many", "one", "other"]);
+  });
+
+  it("leaves out fractional-only forms", () => {
+    expect([...pluralForms("lt")].toSorted()).toEqual(["few", "one", "other"]);
+    expect([...pluralForms("en")].toSorted()).toEqual(["one", "other"]);
+  });
+});