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diff --git a/packages/main/src/i18n/loadLocalization.ts b/packages/main/src/i18n/loadLocalization.ts
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+++ b/packages/main/src/i18n/loadLocalization.ts
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+/*
+ * Copyright (C)  2022 Kristóf Marussy <kristof@marussy.com>
+ *
+ * This file is part of Sophie.
+ *
+ * Sophie 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, version 3.
+ *
+ * This program 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 this program.  If not, see <https://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { FALLBACK_LOCALE, SYSTEM_LOCALE } from '@sophie/shared';
+import i18next, { i18n } from 'i18next';
+import { reaction } from 'mobx';
+import { addDisposer } from 'mobx-state-tree';
+
+import type MainStore from '../stores/MainStore.js';
+import getLogger from '../utils/getLogger.js';
+
+import I18nStore from './I18nStore.js';
+import type LocatlizationRepository from './LocalizationRepository.js';
+import RepositoryBasedI18nBackend from './RepositoryBasedI18nBackend.js';
+import i18nLog from './i18nLog.js';
+
+const log = getLogger('loadLocationzation');
+
+const TEST_LOCALE = 'cimode';
+
+function getLanguage(
+  language: string,
+  systemLocale: string,
+  supportedLanguages: string[],
+): string {
+  const selectedLanguage = language === SYSTEM_LOCALE ? systemLocale : language;
+  if (selectedLanguage === TEST_LOCALE) {
+    return selectedLanguage;
+  }
+  // Even though i18next has a `supportedLngs` array from which it can pick a supported language,
+  // we still have to do this ourselves to avoid spurious warnings like
+  // https://github.com/i18next/i18next/issues/1564
+  if (supportedLanguages.includes(selectedLanguage)) {
+    return selectedLanguage;
+  }
+  if (selectedLanguage.includes('-')) {
+    const iso639 = selectedLanguage.split('-')[0];
+    if (supportedLanguages.includes(iso639)) {
+      return iso639;
+    }
+  }
+  return FALLBACK_LOCALE;
+}
+
+function updateSharedStoreLanguage(store: MainStore, i18nInstance: i18n): void {
+  const resolvedLanguage =
+    i18nInstance.language === TEST_LOCALE
+      ? TEST_LOCALE
+      : i18nInstance.resolvedLanguage;
+  const dir = i18nInstance.dir();
+  // We do not want to pass the list of supported languages to the renderer process,
+  // so we extract the resolved languages from `i18n` as pass only that to the renderer.
+  // Thus, the renderer always selects a language that is actually supported.
+  store.shared.setLanguage(resolvedLanguage, dir);
+  log.debug('Loaded language', resolvedLanguage, 'with direction', dir);
+}
+
+export default async function loadLocalization(
+  store: MainStore,
+  systemLocale: string,
+  supportedLanguages: string[],
+  repository: LocatlizationRepository,
+  devMode: boolean,
+): Promise<void> {
+  const backend = new RepositoryBasedI18nBackend(repository, devMode);
+  const i18nInstance = i18next
+    .createInstance({
+      lng: getLanguage(
+        store.settings.language,
+        systemLocale,
+        supportedLanguages,
+      ),
+      supportedLngs: supportedLanguages,
+      fallbackLng: [FALLBACK_LOCALE],
+      debug: devMode,
+      saveMissing: devMode,
+    })
+    .use(backend)
+    .use(i18nLog);
+  const i18nStore = new I18nStore(i18nInstance);
+  store.setI18n(i18nStore);
+
+  await i18nInstance.init();
+  updateSharedStoreLanguage(store, i18nInstance);
+
+  const disposeChangeLanguage = reaction(
+    () => store.settings.language,
+    (languageSetting) => {
+      (async () => {
+        const languageToSet = getLanguage(
+          languageSetting,
+          systemLocale,
+          supportedLanguages,
+        );
+        if (i18nInstance.language !== languageToSet) {
+          await i18nInstance.changeLanguage(languageToSet);
+          updateSharedStoreLanguage(store, i18nInstance);
+        }
+      })().catch((error) => {
+        log.error('Failed to change language', error);
+      });
+    },
+    {
+      fireImmediately: true,
+    },
+  );
+  addDisposer(store, disposeChangeLanguage);
+}