TL;DR: Yes
The longer version: While developing react native applications, all you do is write JavaScript, unless you're building your own native modules. Your entire application / business logic will be written in JS and not in platform specific languages like Objective-C or Java.
How React Native (RN) works?
RN doesn't convert JS to Objective-C or Java, instead, it uses React to wrap the imperative native UI view system. RN applications leverage the UI libraries of its host platform in Objective-C (for iOS) or Java (for Android), thus rendering the actual native UI components and not just web view.
React Native plugins (native modules)?
The are lot of modules out there and the community is still growing. You'll find modules to get most of the things done. Check https://js.coach/react-native
Do you need to know Objective-C and Java?
No. But it's recommended that you have some knowledge in Obj-C / Java, so that you can customize native module or write your own native modules.