I18nManager.forceRTL doesn't applies changes in first app load

I had the same problem and solved it by invoking forceRTL in MainApplication.java in the onCreate method.

...
import com.facebook.react.modules.i18nmanager.I18nUtil;

...
 @Override
  public void onCreate() {
    super.onCreate();
    SoLoader.init(this, /* native exopackage */ false);

    I18nUtil sharedI18nUtilInstance = I18nUtil.getInstance();
    sharedI18nUtilInstance.forceRTL(this,true);
    sharedI18nUtilInstance.allowRTL(this, true);
  }
...

On IOS add in the AppDelegate.m

...
NSURL *jsCodeLocation; // this probably already exists!
[[RCTI18nUtil sharedInstance] allowRTL:YES];
[[RCTI18nUtil sharedInstance] forceRTL:YES];
...

Source

After a week finally i found a logicly way to solve this issue with using Redux & react-native-restart plugin. I'm also use a nice splash screen to user don't show a restarting progress for this purpose.

So let's dive into code:

Redux action:

export const GET_APP_LAYOUT_DIRECTION = 'GET_APP_LAYOUT_DIRECTION';
export const SET_APP_LAYOUT_DIRECTION = 'SET_APP_LAYOUT_DIRECTION';

export const getAppLayoutDirection = () => ({
  type: GET_APP_LAYOUT_DIRECTION,
});

export const setAppLayoutDirection = direction => ({
  type: SET_APP_LAYOUT_DIRECTION,
  direction
});

Redux Reducer:

import {
    GET_APP_LAYOUT_DIRECTION,
    SET_APP_LAYOUT_DIRECTION,
} from '../actions/app';

const initialState = {
    layout: 'ltr',
};

const reducer = (state = initialState, action) => {
    switch (action.type) {
      case GET_APP_LAYOUT_DIRECTION:
        return {
          ...state,
        };
      case SET_APP_LAYOUT_DIRECTION:
        return {
          ...state,
          layout: action.direction,
        };
      default:
        return state;
    }
};

export default reducer;

Home Screen:

import PropTypes from 'prop-types';
import { connect } from 'react-redux';
import RNRestart from 'react-native-restart'; // Import package from node modules
import { getAppLayoutDirection, setAppLayoutDirection } from '../actions/app';

class Home extends PureComponent {
   constructor(props) {
     super(props);

     this.props.dispatch(getAppLayoutDirection());
     if(this.props.layout === 'ltr'){
       this.props.dispatch(setAppLayoutDirection('rtl'));
       RNRestart.Restart();
     }
  }

  componentDidMount() {
     if(this.props.layout && this.props.layout === 'rtl'){
        SplashScreen.hide();
     }
  }
} 

const mapStateToProps = (state) => {
  const { layout } = state.app;
  return {
    layout
  };
}

export default connect(mapStateToProps)(Home);

I went through the same issue this helped me. This is abit modified answer without the need to use redux.

First you check current state with I18nManager.isRTL then forceRTL if not and restart with react-native-restart.

   constructor(props) {          
      //Force RTL
      if(!I18nManager.isRTL){
         I18nManager.forceRTL(true);
         RNRestart.Restart();
      }
   }