Allow Lingui to use linguiLocale with country identifiers like en-us, it would always load en in this case. Introced a new useLocale hook to use the correct locale string to use in Intl methods.

Allow Lingui to use linguiLocale with country identifiers like en-us, it would always load en in this case. Introced a new useLocale hook to use the correct locale string to use in Intl methods. #5268