require-i18n-default-locale-in-locales

Require i18n.locales to include i18n.defaultLocale when i18n config is declared.

Targeted pattern scope

This rule targets docusaurus.config.* files and inspects the top-level i18n object.

What this rule reports

This rule reports when:

i18n.defaultLocale is missing or statically empty, or
i18n.locales does not include i18n.defaultLocale.

Why this rule exists

Locale routing and fallback behavior assume the default locale is part of the supported locale set. Keeping defaultLocale and locales aligned avoids hard-to-debug locale routing mismatches.

❌ Incorrect

export default {
    i18n: {
        defaultLocale: "en",
        locales: ["fr"],
    },
};

✅ Correct

export default {
    i18n: {
        defaultLocale: "en",
        locales: ["en", "fr"],
    },
};

Behavior and migration notes

This rule autofixes static-array cases by appending defaultLocale to locales.

When locales is identifier-backed but statically resolvable, it reports and provides a suggestion to replace the property value with an explicit array literal that includes defaultLocale.

When not to use it

Do not use this rule if your locale configuration is intentionally dynamic and enforced outside ESLint checks.

Rule catalog ID: R051

Targeted pattern scope​

What this rule reports​

Why this rule exists​

❌ Incorrect​

✅ Correct​

Behavior and migration notes​

When not to use it​

Further reading​