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fluxer_api/src/infrastructure/EmailI18nService.ts

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+/*
+ * Copyright (C) 2026 Fluxer Contributors
+ *
+ * This file is part of Fluxer.
+ *
+ * Fluxer is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Fluxer is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with Fluxer. If not, see <https://www.gnu.org/licenses/>.
+ */
+
+import MessageFormat from '@messageformat/core';
+import {Logger} from '~/Logger';
+import {type EmailTemplateKey, type EmailTemplateVariables, getLocaleTranslations, hasLocale} from './email_i18n';
+
+export interface LocalizedEmailTemplate {
+	subject: string;
+	body: string;
+}
+
+export class EmailI18nService {
+	private readonly defaultLocale = 'en-US';
+	private readonly messageFormatCache: Map<string, MessageFormat> = new Map();
+
+	getTemplate<T extends EmailTemplateKey>(
+		templateKey: T,
+		locale: string | null,
+		variables: EmailTemplateVariables[T],
+	): LocalizedEmailTemplate {
+		const effectiveLocale = this.getEffectiveLocale(locale);
+		const translations = getLocaleTranslations(effectiveLocale);
+		const fallbackTranslations = getLocaleTranslations(this.defaultLocale);
+		const template = translations[templateKey] ?? fallbackTranslations[templateKey];
+		if (!template) {
+			throw new Error(`Missing email template ${templateKey} for locale ${effectiveLocale}`);
+		}
+
+		const subjectMf = this.getMessageFormat(effectiveLocale);
+		const subject = subjectMf.compile(template.subject)(variables);
+
+		const bodyMf = this.getMessageFormat(effectiveLocale);
+		const body = bodyMf.compile(template.body)(variables);
+
+		return {subject, body};
+	}
+
+	formatDate(
+		date: Date,
+		locale: string | null,
+		options: Intl.DateTimeFormatOptions = {dateStyle: 'full', timeStyle: 'short'},
+	): string {
+		const effectiveLocale = this.getEffectiveLocale(locale);
+		return date.toLocaleString(effectiveLocale, options);
+	}
+
+	formatNumber(num: number, locale: string | null): string {
+		const effectiveLocale = this.getEffectiveLocale(locale);
+		return num.toLocaleString(effectiveLocale);
+	}
+
+	private getMessageFormat(locale: string): MessageFormat {
+		if (!this.messageFormatCache.has(locale)) {
+			this.messageFormatCache.set(locale, new MessageFormat(locale));
+		}
+		return this.messageFormatCache.get(locale)!;
+	}
+
+	private getEffectiveLocale(locale: string | null): string {
+		if (!locale) {
+			return this.defaultLocale;
+		}
+
+		if (!hasLocale(locale)) {
+			Logger.warn({locale}, 'Unsupported locale for email, falling back to en-US');
+			return this.defaultLocale;
+		}
+
+		return locale;
+	}
+}