feat(frontend): russian and english interface with a language choice in settings, soon stubs removed

frontend/src/lib/format/datetime.ts

@@ -1,29 +1,39 @@
-const time = new Intl.DateTimeFormat(undefined, {
-  hour: "2-digit",
-  minute: "2-digit",
-});
-const day = new Intl.DateTimeFormat(undefined, {
-  day: "numeric",
-  month: "short",
-});
-const full = new Intl.DateTimeFormat(undefined, {
-  day: "numeric",
-  hour: "2-digit",
-  minute: "2-digit",
-  month: "short",
-  year: "numeric",
-});
+import { locale } from "#lib/i18n/locale.svelte.js";
+
+const OPTIONS = {
+  day: { day: "numeric", month: "short" },
+  full: {
+    day: "numeric",
+    hour: "2-digit",
+    minute: "2-digit",
+    month: "short",
+    year: "numeric",
+  },
+  time: { hour: "2-digit", minute: "2-digit" },
+} satisfies Record<string, Intl.DateTimeFormatOptions>;
+
+const formatters = new Map<string, Intl.DateTimeFormat>();
+
+function format(kind: keyof typeof OPTIONS, date: Date): string {
+  const key = `${locale.tag}:${kind}`;
+  let formatter = formatters.get(key);
+  if (!formatter) {
+    formatter = new Intl.DateTimeFormat(locale.tag, OPTIONS[kind]);
+    formatters.set(key, formatter);
+  }
+  return formatter.format(date);
+}
 
 export function formatTime(iso: string): string {
-  return time.format(new Date(iso));
+  return format("time", new Date(iso));
 }
 
 export function formatDay(iso: string): string {
-  return day.format(new Date(iso));
+  return format("day", new Date(iso));
 }
 
 export function formatFull(iso: string): string {
-  return full.format(new Date(iso));
+  return format("full", new Date(iso));
 }
 
 export function formatListDate(iso: string | null): string {
@@ -33,7 +43,7 @@ export function formatListDate(iso: string | null): string {
   const date = new Date(iso);
   const now = new Date();
   if (date.toDateString() === now.toDateString()) {
-    return time.format(date);
+    return format("time", date);
   }
-  return day.format(date);
+  return format("day", date);
 }