If you are using create-react-app, it would have a main file index.js in the src folder of your project.
This index.js has a line where you would see that they have included the index.css file as follows (3rd line):
import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
This is the method that is suggested to be used if you want to include css files programatically.
You can then modify the index.js code based on a flag for rtl and ltr. For example, you have a flag variable (Say let ltr = true) which has either true or false based on the language you have selected.
Then the code can be:
import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
//Fetch the flag variable's value based on the language selected.
...
//After that variable (say ltr) is fetched do the following code.
if(ltr === true) {
import './ltr.css';
} else {
import './rtl.css';
}
PS. Above code is just to give you an idea, you can manage the flag in redux store, localStorage etc.
Hope this helps.