Supporting Language Switching for Date Formats, Not Just Text, with i18next

This article is an English translation of the original Japanese article.

After introducing i18next to a React Native app, dates like 7月24日(金) remained in Japanese on the screen. Just adding more translation JSON does not change places where Date is directly formatted.

To avoid having text and dates in different languages, I unified the language detection and passed it to Intl.DateTimeFormat.

Three Language Settings

The setting values are Japanese, English, and follow device, for a total of three types.

export type LanguagePreference = "system" | "ja" | "en";

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I save the user’s choice in SecureStore. Only when system, I read the device locale from Hermes’s Intl.

function detectSystemLanguage(): "ja" | "en" {
  const locale = Intl.DateTimeFormat().resolvedOptions().locale;
  return locale.toLowerCase().startsWith("ja") ? "ja" : "en";
}

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In this implementation, I also call i18n.changeLanguage the moment the UI changes.

export async function setLanguagePreference(pref: LanguagePreference) {
  await SecureStore.setItemAsync(LANGUAGE_KEY, pref);
  const language = pref === "system" ? detectSystemLanguage() : pref;
  await i18n.changeLanguage(language);
}

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Not Handwriting Dates

Previously, I held months and weekdays in arrays and assembled strings. I replace this with Intl.DateTimeFormat.

export function formatScheduleDate(value: Date, language: string) {
  const locale = language === "ja" ? "ja-JP" : "en-US";

  return new Intl.DateTimeFormat(locale, {
    month: "long",
    day: "numeric",
    weekday: "short",
  }).format(value);
}

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In components, I use i18n.resolvedLanguage.

const { t, i18n } = useTranslation();
const label = formatScheduleDate(date, i18n.resolvedLanguage ?? "ja");

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I check resolvedLanguage to align with the language i18next actually adopted, including fallbacks.

Gradually Increasing Translation Coverage

Replacing all screens at once means tracking translation gaps and layout breaks simultaneously. I actually proceeded in the following order:

1. Language saving, dashboard, and tabs
2. Main screens for attendance registration
3. Settings, notifications, admin screens, and date displays

By handling date support last, I could verify it separately from text translation gaps. I check if the weekday changed, if the month-day order changed, and if the layout breaks with long English representations.

Counting only strings passing through the translation function does not determine the completion of multi-language support. Displays generated outside JSON, like dates, times, numbers, and error messages, also need to connect to the language setting.

References

• i18next documentation
• MDN: Intl.DateTimeFormat