[huntie]

Summary

Choosing a language other than English or Chinese makes the frontend fetch ./locales/<locale>.json, which React Native never ships: only bundled_devtools_locales (en-US and zh, set upstream in config/gni/i18n.gni) are packaged, and the rest are meant to come from Chrome's remoteBase CDN, which React Native never passes. The dev server logs Error: ENOENT: no such file or directory, stat '…/core/i18n/locales/it.json' on every launch, and the frontend falls back to English only once the failed request comes back.

In the RN DevTools desktop app with a real app attached (RN 0.88.0 on an iOS Simulator, the language setting it, DevTools opened through dev-middleware's /open-debugger), it.json answers 404, Metro logs the ENOENT, and the app warns before falling back to English:

DevTools	Metro

Users get there without looking for it: with the default setting, a system language that DevTools supports shows the "DevTools is now available in …" infobar on first launch, and both of its buttons pick a locale React Native can't load. The Settings language dropdown lists every one of about 80 locales too.

When there is no remote base, build the supported-locale list from the bundled locales only. The language dropdown then offers English and Chinese, the infobar appears only for Chinese, and a stored unbundled locale (or "Browser UI language" on, say, an Italian system) resolves to en-US without a request. With a remote base, every locale is offered as before.

• NOTE: this also applies to this repo's hosted dev mode (npm start), which has no remote base either.

Fixes react/react-native#52461

Test plan

Served the frontend from a static server that answers 404 for missing files, like Metro, and loaded rn_fusebox.html in headless Chrome for Testing with navigator.language set. Before is the published @react-native/debugger-frontend@0.88.0; after is this commit's release GN build (is_debug=false, so no en-XL) with locales other than en-US and zh removed, as in the published package.

• Italian system language, default setting. Before: "DevTools is now available in Italian" with "Always match Chrome's language" and "Switch DevTools to Italian". After: no infobar.

  Before	After

• Settings > Preferences > Language. Before: 83 options. After: "Browser UI language", "English (US)" and "Chinese". Headless Chrome draws no native popup, so each image is the dropdown's own <select> with size set to its option count.

  Before	After

• After, with an Italian system language and the setting stored as it or browserLanguage: only en-US.json is requested, with no 404 and no fallback warning.

• After, with a Chinese system language: the Chinese infobar still shows by default, and with the setting zh or browserLanguage, zh.json loads (200) and the UI renders in Chinese.

• npm test -- front_end/core/i18n: 56 of 56 pass, including two new tests that only bundled locales are offered and resolved without a remote base.

• npm run lint -- front_end/core/i18n/i18nImpl.ts front_end/core/i18n/i18n.test.ts passes.

• Not covered: this commit in the RN DevTools desktop app with a real app attached.

• This change maintains backwards compatibility with previous Local Storage data (if modifying settings, experiments, or other persisted client state).

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo following the contribution guide for Meta employees OR contribution guide.
• This commit is React Native-specific and cannot be upstreamed.