Fix #18: Replace currentLocale with prefered languages

On the iOS device the currentLocale returns the region instead of the device language. By using prefered language this is fixed.

RNI18n/RNI18n.m

@@ -16,7 +16,7 @@
 RCT_EXPORT_MODULE();
 
 -(NSString*) getCurrentLocale{
-    NSString *localeString=[[NSLocale currentLocale] localeIdentifier];
+    NSString *localeString=[[NSLocale preferredLanguages] objectAtIndex:0];
     return localeString;
 }