Fix wrong I18nManager.isRTL value on iOS

[scarlac]

Summary:

Fixes #51647

Changelog:

[iOS] [Fixed] Fix wrong I18nManager.isRTL value on iOS

Test Plan:

Ran this change in English (US, LTR), Hebrew (RTL), and Arabic (RTL) to confirm that they appear correctly.

[scarlac]

Closing. It looks like this is a mix of poor Apple docs and a lack of comments in the code.
isApplicationPreferredLanguageRTL does indeed check for the language correctly, but it does so by intersecting the system preferred language with Xcode project defined "Localizations" aka .pbxproj file knownRegions = ( ....

So by default, I18nManager.isRTL will not work, including in RNTester, since that project does not specify RTL languages:
https://github.com/facebook/react-native/blob/main/packages/rn-tester/RNTesterPods.xcodeproj/project.pbxproj#L466

This is all unexpected, undocumented behavior, so it explains why many issues have been raised regarding the use of .forceRTL(true) not being applied immediately - it's not meant for production use.

[Saadnajmi]

Closing. It looks like this is a mix of poor Apple docs and a lack of comments in the code. isApplicationPreferredLanguageRTL does indeed check for the language correctly, but it does so by intersecting the system preferred language with Xcode project defined "Localizations" aka .pbxproj file knownRegions = ( ....

So by default, I18nManager.isRTL will not work, including in RNTester, since that project does not specify RTL languages: https://github.com/facebook/react-native/blob/main/packages/rn-tester/RNTesterPods.xcodeproj/project.pbxproj#L466

This is all unexpected, undocumented behavior, so it explains why many issues have been raised regarding the use of .forceRTL(true) not being applied immediately - it's not meant for production use.

You coouldd open a new PR to add more comments to the code 😄. Or maybe add more languages to RNTester, but that might be trickier if it messes with CI.

[scarlac]

@Saadnajmi Exactly. I think we also need to add an official docs page as well, as I18nManager is a vital piece of RN's internal rendering, and it's widely being misused.

#51661