Hi matsa .
Welcome to the forums! 🥳
 
You can use the Page Rule URL begins with to make your button appear only on English pages.
Here's a tutorial on how you should set this up.
1. Buttons
You will have to make 2 seperate buttons for both pages.
- 1 French Button
- 1 English Button
  
2. Page Rule
Now you have to make a page rule to specify which page(s) you want to trigger the Page Rule.
Use URL begins with  with the value en/ like this:

3. Make the buttons hide/appear on these pages
Now that you specified where you want to trigger the Page Rule. 
Make the English button only appear on pages with "en/" in the URL.

And make the French button hide on pages with "en/" on the URL.

I hope that this instruction was easy to follow! 🙂
Let me know if this works for you!