In Rad's previous reply, he said switching languages "doesn't reset any content either (with WPML)" and that is the opposite of what's happening on your end. There could be something in your server and/or WordPress setup that is causing the issue.
When I opened your Dutch Home Page in the Content Builder and then switched to the English Page using the Flag Icon, the Content Builder (English version) is empty. I tried loading the NL HOME page template in there and I got the saving success message but it actually didn't save so you still have the testimonial slider in the front-end.
I tested further by creating a new test page. The page I tested was first created in English and then I used the Dutch Flag Icon in the builder and it successfully created the Dutch page however, the issue's still the same. I've edited the Dutch page and the saving success message popped up but it actually didn't save.
The usual cause of this is caching but factors like low memory could also cause this. Would you mind clearing all caches and completely uninstall your caching plugin. Please contact the caching plugin developer to learn how to do that.
Please also deactivate all third party plugins while we investigate so changes we make will take effect immediately.
Also give us FTP access credentials.