Build the two rule descriptions as sentences, not fragments

Both describeRule functions assembled their output by concatenation, which no
catalogue could fix. A translator handed " and " or " on " in isolation cannot
move it: German puts the verb last, Japanese does not separate list items with
a word at all, and the fragments arrive in an order the English sentence chose.
Reported by a native speaker reviewing the German catalogue (#247), whose "the
summaries" item is the Sieve one.

Every branch is now one whole sentence with placeholders, so a translator
rewrites the sentence including its word order. Joining is Intl.ListFormat,
which gives "A, B und C" for an allof rule and the language's own disjunction
for anyof, rather than a hardcoded " and " that would be wrong twice over.

The recurrence tail no longer appends: ", 5 times" and ", until 2026-05-03"
wrap the sentence they qualify, so a language that puts the limit first can.

Ordinals become words. The old suffix table -- st, nd, rd, th, picked by
arithmetic -- is English spelling rules in code, and no catalogue can reach a
suffix chosen that way. German writes "1.", Japanese "第1". nthOfPeriod is 1-5
or -1 in practice, so five words and "last" cover it.

WEEKDAYS is gone. Its long names could have been catalogue entries but its
short ones never could: "T" is Tuesday and Thursday, "S" is Saturday and
Sunday, and a catalogue cannot hold two translations under one key. That was
bad data rather than missing translation, and Intl has every name in every
locale in three widths. lib/datetime.ts gains weekdayName, weekdayNames and
formatList; recurrence.ts keeps WEEKDAY_KEYS for the ordering, which is not a
language question.

Adds the first tests either function has had. Neither had any, and no test
would have caught what was wrong with them, since the English output was
correct -- so these pin the two properties that actually matter: fragments go
through the catalogue, and the joining is Intl's.

32 strings and 9 plural forms are new and land with each language.

Verified: typecheck clean, 1009 tests pass.

web/src/lib/__tests__/describeRules.test.ts

@@ -0,0 +1,93 @@
+/**
+ * The two sentence builders, which had no tests while they were building
+ * English by concatenation -- and no test would have caught the thing wrong
+ * with them, since the English output was correct. These pin the two
+ * properties that matter now: every fragment goes through the catalogue, and
+ * the joining is Intl's rather than a hardcoded " and ".
+ */
+import { describe, expect, it } from "vitest";
+import { describeRule as describeSieve } from "../sieve";
+import { describeRule as describeRecurrence, weekdayOptions } from "../recurrence";
+import { setUiLanguageForFormatting } from "../datetime";
+import { setCatalog } from "../i18n";
+
+describe("sieve describeRule", () => {
+  it("names the header and operator through the catalogue", () => {
+    const s = describeSieve({
+      id: "1", name: "r", join: "allof", enabled: true,
+      tests: [{ type: "header", header: "subject", op: "contains", value: "invoice" }],
+      actions: [{ type: "fileinto", mailbox: "Work" }],
+    } as never);
+    expect(s).toContain("Subject");
+    expect(s).toContain("contains");
+    expect(s).toContain("invoice");
+    expect(s).toContain("Work");
+  });
+
+  it("joins an allof rule as a conjunction and anyof as a disjunction", () => {
+    const base = {
+      id: "1", name: "r", enabled: true,
+      tests: [
+        { type: "header", header: "from", op: "is", value: "a@b" },
+        { type: "header", header: "to", op: "is", value: "c@d" },
+      ],
+      actions: [{ type: "keep" }],
+    };
+    expect(describeSieve({ ...base, join: "allof" } as never)).toContain(" and ");
+    expect(describeSieve({ ...base, join: "anyof" } as never)).toContain(" or ");
+  });
+
+  it("says 'always' when a rule has no tests", () => {
+    const s = describeSieve({ id: "1", name: "r", join: "allof", enabled: true, tests: [], actions: [{ type: "stop" }] } as never);
+    expect(s).toContain("always");
+  });
+});
+
+describe("recurrence describeRule", () => {
+  it("describes the simple frequencies", () => {
+    expect(describeRecurrence(undefined)).toBe("Does not repeat");
+    expect(describeRecurrence({ "@type": "RecurrenceRule", frequency: "daily" } as never)).toBe("Daily");
+    expect(describeRecurrence({ "@type": "RecurrenceRule", frequency: "daily", interval: 3 } as never)).toBe("Every 3 days");
+  });
+
+  it("recognises Monday to Friday as every weekday", () => {
+    const rule = {
+      "@type": "RecurrenceRule", frequency: "weekly",
+      byDay: ["mo", "tu", "we", "th", "fr"].map((day) => ({ "@type": "NDay", day })),
+    };
+    expect(describeRecurrence(rule as never)).toBe("Every weekday");
+  });
+
+  it("uses a word, not a suffix, for the nth weekday of a month", () => {
+    const s = describeRecurrence({
+      "@type": "RecurrenceRule", frequency: "monthly",
+      byDay: [{ "@type": "NDay", day: "tu", nthOfPeriod: 2 }],
+    } as never);
+    expect(s).toContain("second");
+    expect(s).not.toContain("2nd");
+  });
+
+  it("wraps the sentence for count and until rather than appending to it", () => {
+    const s = describeRecurrence({ "@type": "RecurrenceRule", frequency: "daily", count: 5 } as never);
+    expect(s).toBe("Daily, 5 times");
+    const u = describeRecurrence({ "@type": "RecurrenceRule", frequency: "daily", until: "2026-05-03T00:00:00" } as never);
+    expect(u).toBe("Daily, until 2026-05-03");
+  });
+
+  it("takes its weekday names from the locale, not a table of English", () => {
+    setUiLanguageForFormatting("de-DE");
+    const names = weekdayOptions().map((w) => w.label);
+    expect(names[0]).toBe("Montag");
+    expect(names).toHaveLength(7);
+    // The narrow forms collide in English ("T" for both Tuesday and Thursday),
+    // which is why they cannot be catalogue keys and come from Intl instead.
+    expect(weekdayOptions().map((w) => w.short)).toHaveLength(7);
+    setUiLanguageForFormatting(null);
+  });
+
+  it("renders a translated rule through the catalogue", () => {
+    setCatalog("de", { strings: { Daily: "Täglich" }, plurals: {} });
+    expect(describeRecurrence({ "@type": "RecurrenceRule", frequency: "daily" } as never)).toBe("Täglich");
+    setCatalog("en", { strings: {}, plurals: {} });
+  });
+});

web/src/lib/datetime.ts

@@ -557,3 +557,48 @@ export function localeOptions(): LocaleOption[] {
   optionsExtras = extras;
   return list;
 }
+
+/**
+ * Weekday names in the reader's locale, indexed by JSCalendar's two-letter day.
+ *
+ * These used to be a table of English strings with a `short` of "M", "T", "W"…
+ * which could not become catalogue entries at all: "T" is both Tuesday and
+ * Thursday and "S" is both Saturday and Sunday, so the key collides with
+ * itself. A catalogue cannot hold two translations under one key, and no
+ * amount of translating fixes that — the data was wrong, not the wiring.
+ *
+ * Intl has the names already, in every locale, in three widths, and gets the
+ * plural and capitalisation conventions right without anybody maintaining a
+ * list. 2026-06-01 is a Monday; the rest follow from it.
+ */
+export type WeekdayKey = "mo" | "tu" | "we" | "th" | "fr" | "sa" | "su";
+
+const WEEKDAY_ORDER: WeekdayKey[] = ["mo", "tu", "we", "th", "fr", "sa", "su"];
+const WEEKDAY_BASE = Date.UTC(2026, 5, 1); // a Monday
+
+export function weekdayName(day: WeekdayKey, width: "long" | "short" | "narrow" = "long"): string {
+  const i = WEEKDAY_ORDER.indexOf(day);
+  if (i < 0) return day;
+  return intl({ weekday: width, timeZone: "UTC" }).format(new Date(WEEKDAY_BASE + i * 86_400_000));
+}
+
+/** Every weekday, Monday first, for pickers that show all seven. */
+export function weekdayNames(width: "long" | "short" | "narrow" = "long"): Array<{ key: WeekdayKey; name: string }> {
+  return WEEKDAY_ORDER.map((key) => ({ key, name: weekdayName(key, width) }));
+}
+
+/**
+ * "A, B and C" — or "A, B oder C", or the comma the locale actually uses.
+ *
+ * Joining with a translated " and " does not work: Japanese does not separate
+ * list items with a word, and the last separator differs from the others in
+ * English. Intl.ListFormat knows all of that.
+ */
+export function formatList(items: string[], type: "conjunction" | "disjunction" = "conjunction"): string {
+  if (items.length < 2) return items[0] ?? "";
+  try {
+    return new Intl.ListFormat(resolvedLocale(), { style: "long", type }).format(items);
+  } catch {
+    return items.join(", ");
+  }
+}

web/src/lib/recurrence.ts

@@ -1,14 +1,25 @@
 import type { JSCalendarRecurrenceRule, JSCalendarNDay } from "@/jmap/types";
+import { formatList, weekdayName, weekdayNames } from "./datetime";
+import { plural, t } from "@/lib/i18n";
 
-export const WEEKDAYS: Array<{ key: JSCalendarNDay["day"]; label: string; short: string }> = [
-  { key: "mo", label: "Monday", short: "M" },
-  { key: "tu", label: "Tuesday", short: "T" },
-  { key: "we", label: "Wednesday", short: "W" },
-  { key: "th", label: "Thursday", short: "T" },
-  { key: "fr", label: "Friday", short: "F" },
-  { key: "sa", label: "Saturday", short: "S" },
-  { key: "su", label: "Sunday", short: "S" },
-];
+/**
+ * The seven days, Monday first, named in the reader's locale.
+ *
+ * This was a table of English strings carrying `label: "Monday"` and
+ * `short: "M"`, rendered straight into the picker. The long names could have
+ * become catalogue entries; the short ones could not, because "T" is both
+ * Tuesday and Thursday and "S" is both Saturday and Sunday, and a catalogue
+ * cannot hold two translations under one key. Intl knows all of them.
+ */
+export const WEEKDAY_KEYS: Array<JSCalendarNDay["day"]> = ["mo", "tu", "we", "th", "fr", "sa", "su"];
+
+export function weekdayOptions(): Array<{ key: JSCalendarNDay["day"]; label: string; short: string }> {
+  return weekdayNames("long").map(({ key, name }) => ({
+    key: key as JSCalendarNDay["day"],
+    label: name,
+    short: weekdayName(key, "narrow"),
+  }));
+}
 
 export type RecurrencePreset = "none" | "daily" | "weekly" | "weekdays" | "monthly" | "yearly" | "custom";
 
@@ -28,7 +39,7 @@ export function presetFor(rule: JSCalendarRecurrenceRule | undefined): Recurrenc
 }
 
 export function ruleFromPreset(preset: RecurrencePreset, start: Date): JSCalendarRecurrenceRule | undefined {
-  const dow = WEEKDAYS[(start.getDay() + 6) % 7]!.key;
+  const dow = WEEKDAY_KEYS[(start.getDay() + 6) % 7]!;
   switch (preset) {
     case "daily":
       return { "@type": "RecurrenceRule", frequency: "daily" };
@@ -45,48 +56,107 @@ export function ruleFromPreset(preset: RecurrencePreset, start: Date): JSCalenda
   }
 }
 
+/**
+ * A recurrence rule as a sentence.
+ *
+ * Built as whole sentences with placeholders rather than by concatenation.
+ * The old version appended fragments -- `base += " on " + names` -- which is
+ * untranslatable however complete the catalogue is: German puts the weekday
+ * list somewhere else in the clause, and a translator handed " on " alone
+ * cannot move it. Every branch below is one key a translator can rewrite in
+ * full, including the word order.
+ */
 export function describeRule(rule: JSCalendarRecurrenceRule | undefined): string {
-  if (!rule) return "Does not repeat";
+  if (!rule) return t("Does not repeat");
   const n = rule.interval ?? 1;
+  const every = n !== 1;
   let base: string;
+
   switch (rule.frequency) {
     case "daily":
-      base = n === 1 ? "Daily" : `Every ${n} days`;
+      base = every ? plural(n, { one: "Every {n} day", other: "Every {n} days" }) : t("Daily");
       break;
+
     case "weekly": {
-      base = n === 1 ? "Weekly" : `Every ${n} weeks`;
-      if (rule.byDay?.length) {
-        const names = rule.byDay.map((d) => WEEKDAYS.find((w) => w.key === d.day)?.label ?? d.day);
-        const set = rule.byDay.map((d) => d.day).sort().join(",");
-        if (set === ["mo", "tu", "we", "th", "fr"].sort().join(",") && n === 1) base = "Every weekday";
-        else base += ` on ${names.join(", ")}`;
+      const days = rule.byDay?.length ? rule.byDay.map((d) => d.day) : [];
+      const weekdaysOnly =
+        days.length === 5 && ["mo", "tu", "we", "th", "fr"].every((d) => days.includes(d as JSCalendarNDay["day"]));
+      if (weekdaysOnly && !every) {
+        base = t("Every weekday");
+      } else if (days.length) {
+        const list = formatList(days.map((d) => weekdayName(d as never)));
+        base = every
+          ? plural(n, { one: "Every {n} week on {days}", other: "Every {n} weeks on {days}" }, { days: list })
+          : t("Weekly on {days}", { days: list });
+      } else {
+        base = every ? plural(n, { one: "Every {n} week", other: "Every {n} weeks" }) : t("Weekly");
       }
       break;
     }
+
     case "monthly": {
-      base = n === 1 ? "Monthly" : `Every ${n} months`;
-      if (rule.byMonthDay?.length) base += ` on day ${rule.byMonthDay.join(", ")}`;
-      else if (rule.byDay?.length) {
+      if (rule.byMonthDay?.length) {
+        const list = formatList(rule.byMonthDay.map(String));
+        base = every
+          ? plural(n, { one: "Every {n} month on day {days}", other: "Every {n} months on day {days}" }, { days: list })
+          : t("Monthly on day {days}", { days: list });
+      } else if (rule.byDay?.length) {
         const d = rule.byDay[0]!;
-        const ord = d.nthOfPeriod ? ordinal(d.nthOfPeriod) + " " : "";
-        base += ` on the ${ord}${WEEKDAYS.find((w) => w.key === d.day)?.label ?? d.day}`;
+        const weekday = weekdayName(d.day as never);
+        if (d.nthOfPeriod) {
+          const ord = ordinal(d.nthOfPeriod);
+          base = every
+            ? plural(n, { one: "Every {n} month on the {ordinal} {weekday}", other: "Every {n} months on the {ordinal} {weekday}" }, { ordinal: ord, weekday })
+            : t("Monthly on the {ordinal} {weekday}", { ordinal: ord, weekday });
+        } else {
+          base = every
+            ? plural(n, { one: "Every {n} month on {weekday}", other: "Every {n} months on {weekday}" }, { weekday })
+            : t("Monthly on {weekday}", { weekday });
+        }
+      } else {
+        base = every ? plural(n, { one: "Every {n} month", other: "Every {n} months" }) : t("Monthly");
       }
       break;
     }
+
     case "yearly":
-      base = n === 1 ? "Yearly" : `Every ${n} years`;
+      base = every ? plural(n, { one: "Every {n} year", other: "Every {n} years" }) : t("Yearly");
       break;
+
     default:
-      base = `Every ${n} ${rule.frequency}`;
+      // An RFC frequency this build has no sentence for. The frequency word
+      // itself stays as the server sent it rather than being invented.
+      base = t("Every {n} {frequency}", { n, frequency: rule.frequency });
+  }
+
+  // The tail wraps the sentence rather than being glued to its end, so a
+  // translator can put "until 3 May" first if that is what the language does.
+  if (rule.count) {
+    base = plural(rule.count, { one: "{rule}, {n} time", other: "{rule}, {n} times" }, { rule: base });
+  }
+  if (rule.until) {
+    base = t("{rule}, until {date}", { rule: base, date: rule.until.slice(0, 10) });
   }
-  if (rule.count) base += `, ${rule.count} times`;
-  if (rule.until) base += `, until ${rule.until.slice(0, 10)}`;
   return base;
 }
 
+/**
+ * "first", "second", "last" -- words, not "1st".
+ *
+ * The suffix table this replaced ("st", "nd", "rd", "th") is English spelling
+ * rules in code: German writes "1.", Japanese "第1", and no catalogue can
+ * reach a suffix chosen by arithmetic. JSCalendar's nthOfPeriod is 1-5 or -1
+ * in practice, so five words and "last" cover it; anything else falls back to
+ * the bare number, which is wrong in no language.
+ */
 function ordinal(n: number): string {
-  if (n === -1) return "last";
-  const s = ["th", "st", "nd", "rd"];
-  const v = n % 100;
-  return n + (s[(v - 20) % 10] ?? s[v] ?? s[0]!);
+  switch (n) {
+    case -1: return t("last");
+    case 1: return t("first");
+    case 2: return t("second");
+    case 3: return t("third");
+    case 4: return t("fourth");
+    case 5: return t("fifth");
+    default: return String(n);
+  }
 }

web/src/lib/sieve.ts

@@ -6,6 +6,9 @@
  * Sieve below each comment is what the server actually runs.
  */
 
+import { formatList } from "./datetime";
+import { t } from "@/lib/i18n";
+
 export type HeaderOp = "contains" | "notcontains" | "is" | "notis" | "matches" | "notmatches" | "regex" | "notregex" | "exists" | "notexists";
 
 export type SieveTest =
@@ -318,49 +321,60 @@ export function reorderRules(rules: SieveRule[], fromId: string, toId: string, b
   return [...rest.slice(0, at), moved, ...rest.slice(at)];
 }
 
+/**
+ * A filter rule as a sentence, for the rule list.
+ *
+ * Rebuilt as whole sentences with placeholders. The old version concatenated
+ * fragments -- a header name, an operator, a quoted value, joined by " and "
+ * -- which no catalogue could fix: German puts the verb last, Japanese does
+ * not separate list items with a word at all, and a translator handed " and "
+ * on its own cannot move anything. Reported by a native speaker reviewing the
+ * German catalogue (#247).
+ *
+ * Intl.ListFormat does the joining, so "A, B and C" becomes "A, B und C" and,
+ * for an anyof rule, the disjunction the language actually uses.
+ */
 export function describeRule(r: SieveRule): string {
-  const tests = r.tests
-    .map((t) => {
-      switch (t.type) {
-        case "header":
-          return `${t.header} ${HEADER_OPS.find((o) => o.value === t.op)?.label ?? t.op} "${t.value}"`;
-        case "address":
-          return `${t.header} address ${HEADER_OPS.find((o) => o.value === t.op)?.label ?? t.op} "${t.value}"`;
-        case "size":
-          return `size ${t.op} ${Math.round(t.value / 1024)} KB`;
-        case "body":
-          return `body ${t.op === "contains" ? "contains" : "does not contain"} "${t.value}"`;
-        case "true":
-          return "always";
-      }
-    })
-    .join(r.join === "allof" ? " and " : " or ");
-  const actions = r.actions
-    .map((a) => {
-      switch (a.type) {
-        case "fileinto":
-          return `move to ${a.mailbox}`;
-        case "redirect":
-          return `forward to ${a.address}`;
-        case "discard":
-          return "delete";
-        case "keep":
-          return "keep";
-        case "reject":
-          return "reject";
-        case "markread":
-          return "mark read";
-        case "flag":
-          return "star";
-        case "addflag":
-        case "setflag":
-          return `add ${a.flag}`;
-        case "removeflag":
-          return `remove ${a.flag}`;
-        case "stop":
-          return "stop";
-      }
-    })
-    .join(", ");
-  return `${tests || "always"} → ${actions}`;
+  const headerLabel = (h: string): string => t(HEADER_CHOICES.find((c) => c.value === h)?.label ?? h);
+  const opLabel = (op: string): string => t(HEADER_OPS.find((o) => o.value === op)?.label ?? op);
+
+  const tests = r.tests.map((test) => {
+    switch (test.type) {
+      case "header":
+        return t('{header} {op} "{value}"', { header: headerLabel(test.header), op: opLabel(test.op), value: test.value });
+      case "address":
+        return t('{header} address {op} "{value}"', { header: headerLabel(test.header), op: opLabel(test.op), value: test.value });
+      case "size":
+        return test.op === "over"
+          ? t("size is over {n} KB", { n: Math.round(test.value / 1024) })
+          : t("size is under {n} KB", { n: Math.round(test.value / 1024) });
+      case "body":
+        return test.op === "contains"
+          ? t('body contains "{value}"', { value: test.value })
+          : t('body does not contain "{value}"', { value: test.value });
+      case "true":
+        return t("always");
+    }
+  });
+
+  const actions = r.actions.map((a) => {
+    switch (a.type) {
+      case "fileinto":   return t("move to {folder}", { folder: a.mailbox });
+      case "redirect":   return t("forward to {address}", { address: a.address });
+      case "discard":    return t("delete it");
+      case "keep":       return t("keep it");
+      case "reject":     return t("reject it");
+      case "markread":   return t("mark it read");
+      case "flag":       return t("star it");
+      case "addflag":
+      case "setflag":    return t("add {flag}", { flag: a.flag });
+      case "removeflag": return t("remove {flag}", { flag: a.flag });
+      case "stop":       return t("stop");
+    }
+  });
+
+  return t("{tests} → {actions}", {
+    tests: tests.length ? formatList(tests, r.join === "allof" ? "conjunction" : "disjunction") : t("always"),
+    actions: formatList(actions, "conjunction"),
+  });
 }

web/src/views/calendar/EventEditor.tsx

@@ -13,7 +13,7 @@ import { RecipientInput } from "../compose/RecipientInput";
 import { DateField, DateTimeField } from "@/ui/datefield";
 import { browserTimeZone, dateToZonedLocal, formatDuration, fromInputDateTime, listTimeZones, parseDuration, toInputDateTime, toLocalDateOnly, zonedToDate, DAY_MS, humanDuration } from "@/lib/dates";
 import { formatClock, formatNumericDate, formatWeekday, formatWeekdayDate } from "@/lib/datetime";
-import { WEEKDAYS, describeRule, presetFor, ruleFromPreset, type RecurrencePreset } from "@/lib/recurrence";
+import { WEEKDAY_KEYS, weekdayOptions, describeRule, presetFor, ruleFromPreset, type RecurrencePreset } from "@/lib/recurrence";
 import { newKey } from "@/lib/contacts";
 import { availabilityWindow } from "@/lib/availabilityWindow";
 import { askEditScope, droppedMessage, runScoped } from "./scope";
@@ -359,7 +359,7 @@ function EventForm({ init, base, scope, editing, onClose, settingsTz, defaultAle
             </select>
           )}
           {!oneDate && (
-          <select className="select" style={{ width: "auto", height: 32 }} value={preset} onChange={(e) => { const p = e.target.value as RecurrencePreset; setPreset(p); if (p === "custom") setRule(rule ?? { "@type": "RecurrenceRule", frequency: "weekly", byDay: [{ "@type": "NDay", day: WEEKDAYS[(start.getDay() + 6) % 7]!.key }] }); else setRule(ruleFromPreset(p, start)); }}>
+          <select className="select" style={{ width: "auto", height: 32 }} value={preset} onChange={(e) => { const p = e.target.value as RecurrencePreset; setPreset(p); if (p === "custom") setRule(rule ?? { "@type": "RecurrenceRule", frequency: "weekly", byDay: [{ "@type": "NDay", day: WEEKDAY_KEYS[(start.getDay() + 6) % 7]! }] }); else setRule(ruleFromPreset(p, start)); }}>
             <option value="none">{translate("Does not repeat")}</option>
             <option value="daily">{translate("Daily")}</option>
             <option value="weekly">{translate("Weekly on {weekday}", { weekday: formatWeekday(start, "long") })}</option>
@@ -381,7 +381,7 @@ function EventForm({ init, base, scope, editing, onClose, settingsTz, defaultAle
             </div>
             {customRule.frequency === "weekly" && (
               <div className="row" style={{ gap: 4, marginTop: 8 }}>
-                {WEEKDAYS.map((w) => {
+                {weekdayOptions().map((w) => {
                   const on = customRule.byDay?.some((d) => d.day === w.key);
                   return <button key={w.key} type="button" className={`btn btn-sm btn-pill ${on ? "btn-primary" : ""}`} style={{ width: 36, padding: 0 }} title={w.label} onClick={() => { const cur = customRule.byDay ?? []; const next: JSCalendarNDay[] = on ? cur.filter((d) => d.day !== w.key) : [...cur, { "@type": "NDay", day: w.key }]; setRule({ ...customRule, byDay: next.length ? next : undefined }); }}>{w.short}</button>;
                 })}