Is React Native made for windows phone too? If no what is best language or tool that is worth learning in mobile app cross-platform development?
React Native doesn't come with support for Windows Phone (7/8/8.1).
Microsoft has created a really good React Native plugin for Windows Phone's successor: Windows 10 Mobile, but they also announced that the company no longer focuses on the platform so choose wisely.
React Native Windows plugin repository: https://github.com/Microsoft/react-native-windows