fix(i18n): fix locale detection for regional variants on React Native

[Beautiful-blue-sky]

Summary

Fix locale detection for regional language variants on React Native (Android & iOS).

Problem

The detect() function in languageDetector.native.ts was building locale strings by directly concatenating language and region codes without a separator. This produced invalid keys like zhCN instead of zh-CN, which never matched any entry in languages.json, causing the app to fall back to English — even when the system language was correctly set.

Affected locales: zh-CN, zh-TW, fr-CA, pt-BR, es-US, and any other regional variants.

Root cause introduced in commit 9be78c6 and carried over in 35c7f15.

Fix

Insert - separator when constructing the locale string:

System locale	Before	After
zh-CN (Android)	zhCN ❌	zh-CN ✅
zh-Hans-CN (iOS)	zhCN ❌	zh-CN ✅
pt-BR	ptBR ❌	pt-BR ✅
fr-CA	frCA ❌	fr-CA ✅

Testing

Verified on Android emulator (API 36, locale zh-Hans-CN):

• Before fix: Settings showed Language: zh (invalid fallback), UI displayed in English
• After fix: Settings showed Language: 中文（简体）, UI displayed in Simplified Chinese

Also verified on physical Android device with Simplified Chinese system language.

[Copilot]

Pull request overview

Fixes React Native locale detection so regional variants match languages.json keys (e.g., zh-CN, pt-BR) instead of incorrectly concatenated strings (e.g., zhCN) that caused unintended fallback to English.

Changes:

• Construct BCP-47 style locale strings with a - separator for both lang-region and lang-script-region inputs.
• Add clarifying inline examples for iOS (zh-Hans-CN) and Android (zh-CN) locale formats.

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[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[Beautiful-blue-sky]

Hi, thanks for your contribution! If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

Thank you! I have signed the Individual CLA.