Flutter — is written on Dart.
Dart is C family language so if you know Java, JavaScript, Kotlin etc you will not have problems to learn Dart.
Flutter is similar to RN from point of view writing markup. They are both use a declarative approach. But in Flutter you are writing all markup in Dart. You need to know Swift or Kotlin/Java only if you want to write a connector for some Native API (like Camera or Battery API) let’s call it “logic” API and that only if no one else written it before. In RN if you want something custom view(widget) you should implement it three times (Swift, Kotlin, JS) and it’s not so unusual situation the same for “logic” API.

DX(Developer Experience) is also better in Flutter. Good documentation, robust test infrastructure, strong team, and community, the fast feedback loop(hot reload/restart), good integration with IDEs, types out of the box.