The downgrade fixes the builder issue, the French version is now loading within the builder. BUT, it's completely empty, I assume it was translated this way https://www.youtube.com/watch?v=TmmyU6MtXY8. Where it only search visible strings and replace it. And it was not really translated within the builder before. And it seems like it based on the existing old content.
For that, I recommend translating it within the builder. Example, switch to English page and save it as a Template, then switch to French page and import the saved English template. Then edit it and translate the content of the French page.
This is an old video for old cornerstone, and it should be translated that way even before https://www.dropbox.com/s/5oq0a5f7s2fd852/CornerstoneTranslation.mp4?dl=0
I only requested the downgrade since the builder wasn't loading.