Connect the OS language with isRTL

Summary: Create isDeviceLanguageRTL function to connect the OS language with isRTL. Now even if a new app could support RTL language, without setting forceRTL at JS side it won't directly change into RTL layout.

Reviewed By: fkgozali

Differential Revision: D3473109

fbshipit-source-id: ac1410c910e980d44750bb88cad7615febb9e076

React/Modules/RCTI18nUtil.m

@@ -24,7 +24,9 @@
 
 - (BOOL)isRTL
 {
-  if ([self forceRTL]) return YES;
+  if ([self forceRTL] && [self isDevicePreferredLanguageRTL]) {
+    return YES;
+  }
   return NO;
 }
 
@@ -41,4 +43,10 @@
   [[NSUserDefaults standardUserDefaults] synchronize];
 }
 
+- (BOOL)isDevicePreferredLanguageRTL
+{
+  NSLocaleLanguageDirection direction = [NSLocale characterDirectionForLanguage:[[NSLocale preferredLanguages] objectAtIndex:0]];
+  return direction == NSLocaleLanguageDirectionRightToLeft;
+}
+
 @end