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

frontend/src/lib/i18n/locale.svelte.ts

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+import { type MessageKey, messages } from "#lib/i18n/messages.js";
+import type { Locale, Text } from "#lib/i18n/types.js";
+import { browser } from "$app/env";
+
+type Preference = Locale | "system";
+type Params = Record<string, number | string>;
+
+const STORAGE_KEY = "bg.locale";
+const PARAM = /\{(\w+)\}/g;
+const TAGS: Record<Locale, string> = { en: "en-GB", ru: "ru-RU" };
+const plurals: Record<Locale, Intl.PluralRules> = {
+  en: new Intl.PluralRules(TAGS.en),
+  ru: new Intl.PluralRules(TAGS.ru),
+};
+
+function readPreference(): Preference {
+  if (!browser) {
+    return "system";
+  }
+  const stored = localStorage.getItem(STORAGE_KEY);
+  return stored === "en" || stored === "ru" ? stored : "system";
+}
+
+function systemLocale(): Locale {
+  if (!browser) {
+    return "ru";
+  }
+  for (const language of navigator.languages) {
+    const base = language.toLowerCase().slice(0, 2);
+    if (base === "ru" || base === "en") {
+      return base;
+    }
+  }
+  return "en";
+}
+
+function createLocale() {
+  let preference = $state<Preference>(readPreference());
+  const current = $derived<Locale>(
+    preference === "system" ? systemLocale() : preference
+  );
+
+  return {
+    get current() {
+      return current;
+    },
+    get preference() {
+      return preference;
+    },
+    set(value: Preference) {
+      preference = value;
+      if (browser) {
+        localStorage.setItem(STORAGE_KEY, value);
+      }
+    },
+    get tag() {
+      return TAGS[current];
+    },
+  };
+}
+
+export const locale = createLocale();
+
+function pick(text: Text, count: number | undefined): string {
+  if (typeof text === "string") {
+    return text;
+  }
+  return text[plurals[locale.current].select(count ?? 0)] ?? text.other;
+}
+
+export function t(key: MessageKey, params?: Params): string {
+  const count = typeof params?.count === "number" ? params.count : undefined;
+  const text = pick(messages[key][locale.current], count);
+  if (!params) {
+    return text;
+  }
+  return text.replace(PARAM, (match, name: string) =>
+    name in params ? String(params[name]) : match
+  );
+}

frontend/src/lib/i18n/messages.ts

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+import { common } from "#lib/i18n/messages/common.js";
+import { settings } from "#lib/i18n/messages/settings.js";
+
+export const messages = {
+  ...common,
+  ...settings,
+};
+
+export type MessageKey = keyof typeof messages;

frontend/src/lib/i18n/messages/common.ts

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+import type { Messages } from "#lib/i18n/types.js";
+
+export const common = {
+  "common.back": { en: "Back", ru: "Назад" },
+  "common.cancel": { en: "Cancel", ru: "Отмена" },
+  "common.close": { en: "Close", ru: "Закрыть" },
+  "common.delete": { en: "Delete", ru: "Удалить" },
+  "common.save": { en: "Save", ru: "Сохранить" },
+} satisfies Messages;

frontend/src/lib/i18n/messages/settings.ts

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+import type { Messages } from "#lib/i18n/types.js";
+
+export const settings = {
+  "settings.about.license": { en: "GPL-3.0 license", ru: "Лицензия GPL-3.0" },
+  "settings.about.port": {
+    en: "UI ported from Telegram A",
+    ru: "UI портирован из Telegram A",
+  },
+  "settings.about.version": { en: "Version {version}", ru: "Версия {version}" },
+  "settings.alerts": { en: "Alerts", ru: "Оповещения" },
+  "settings.appearance": { en: "Appearance", ru: "Внешний вид" },
+  "settings.bots": { en: "Bots", ru: "Боты" },
+  "settings.jobs": { en: "Backfill and jobs", ru: "Бэкфилл и задачи" },
+  "settings.language": { en: "Language", ru: "Язык" },
+  "settings.language.system": { en: "System", ru: "Как в системе" },
+  "settings.policy": { en: "Capture policy", ru: "Политика захвата" },
+  "settings.shares": { en: "Files by link", ru: "Файлы по ссылке" },
+  "settings.storage": { en: "Storage", ru: "Хранилище" },
+  "settings.stories": { en: "Stories", ru: "Сторис" },
+  "settings.theme.dark": { en: "Dark", ru: "Тёмная" },
+  "settings.theme.light": { en: "Light", ru: "Светлая" },
+  "settings.theme.system": { en: "System", ru: "Системная" },
+  "settings.title": { en: "Settings", ru: "Настройки" },
+  "settings.watches": { en: "Watches", ru: "Наблюдения" },
+} satisfies Messages;

frontend/src/lib/i18n/types.ts

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+export type Locale = "en" | "ru";
+
+export type Plural = Partial<Record<Intl.LDMLPluralRule, string>> & {
+  other: string;
+};
+
+export type Text = string | Plural;
+
+export type Messages = Record<string, Record<Locale, Text>>;