Our 2nd tier checked the issue and suggested that this might be happening due to server-side cache, so can you please disable any cache available on the site and try to translate the page?
I consulted with our hosting service (WP Engine) and the cache is by default disabled in the WP Admin area. So this could not be the issue. You already spotted the error with all plugins disabled so is not an external plugin either.
Here is more information on WP Engine cache. hidden link