feat(i18n): add i18n module for localization support

- Introduce functions to initialize and load locale files dynamically
- Add support for translation keys with placeholder replacements
- Default to English if locale files are missing or not found
- Determine system language using environment variables

src/i18n/i18n.ts

@@ -0,0 +1,74 @@
+/**
+ * Initializes the i18n module by loading
+ * the appropriate locale file based on the system language.
+ */
+export async function initI18n(): Promise<void> {
+    await loadLocale(getCurrentLanguage());
+}
+
+/**
+ * Contains the loaded translations for the current language.
+ * The keys represent i18n identifiers, the values are the localized strings.
+ */
+let translations: Record<string, string> = {};
+
+/**
+ * Loads the translation file for the specified locale.
+ *
+ * Expects a JSON file in the same directory named like `de.json` or `en.json`.
+ * Falls back to English ('en') if the specified file does not exist.
+ *
+ * @param locale - The language code (e.g., 'de', 'en') to load
+ * @returns Promise that resolves once the translations have been loaded
+ */
+export async function loadLocale(locale: string): Promise<void> {
+    try {
+        const localeUrl = new URL(`./${locale}.json`, import.meta.url);
+        const file = await Deno.readTextFile(localeUrl);
+        translations = JSON.parse(file);
+    } catch (err) {
+        if (err instanceof Deno.errors.NotFound) {
+            console.warn(
+                `Locale '${locale}' not found – falling back to 'en'.`,
+            );
+            if (locale !== 'en') {
+                await loadLocale('en');
+            }
+        } else {
+            console.error('Error loading translation file:', err);
+        }
+    }
+}
+
+/**
+ * Translates a given key using the loaded translation data.
+ * Replaces placeholders in the format `{variable}` with the provided values.
+ *
+ * @param key - The i18n key (e.g., 'timer_created')
+ * @param vars - Optional replacements for placeholders in the string
+ * @returns The translated string, or the key itself if no translation is found
+ */
+export function t(key: string, vars: Record<string, string> = {}): string {
+    let text = translations[key] ?? key;
+    for (const [k, v] of Object.entries(vars)) {
+        text = text.replace(`{${k}}`, v);
+    }
+    return text;
+}
+
+/**
+ * Determines the current language from the system environment.
+ *
+ * Priority: SYSTEMD_TIMER_LANG > LC_ALL > LC_MESSAGES > LANG.
+ *
+ * @returns The language code (e.g., 'de', 'en'), defaults to 'en'
+ */
+export function getCurrentLanguage(): string {
+    return (
+        Deno.env.get('SYSTEMD_TIMER_LANG') ??
+            Deno.env.get('LC_ALL') ??
+            Deno.env.get('LC_MESSAGES') ??
+            Deno.env.get('LANG') ??
+            'en'
+    ).split('.')[0].split('_')[0].toLowerCase();
+}

src/i18n/mod.ts

@@ -0,0 +1 @@
+export { getCurrentLanguage, initI18n, loadLocale, t } from './i18n.ts';