This thread is resolved. Here is a description of the problem and solution.
Problem: The client was experiencing an issue with the WPML language switcher being inconsistent with the themes installed. Solution: We found that the problem was related to the "Use menu cache" option in the ThemeREX Addons Settings. We recommended unchecking this option to resolve the issue. If you're experiencing a similar problem, navigate to your ThemeREX Addons Settings and uncheck the "Use menu cache" option.
We apologize for the inconvenience, but in order to proceed with troubleshooting, we kindly request a site snapshot. Alternatively, with your permission, we can create a snapshot on our own.
For this purpose, we typically recommend using the free plugin "Duplicator." If you are already familiar with how Duplicator works ( https://wordpress.org/plugins/duplicator/), please skip the following steps and simply send me the archive file you downloaded.
To assist you further, please follow these instructions:
Watch the Duplicator instructions video: hidden link
If the archive file exceeds 400MB in size, please utilize Duplicator's file filters to exclude the cache, wp-uploads directory, media, and archive files.
Once you have the archive file, please share the link with us. You can use services such as Google Drive, Dropbox, or similar platforms as the snapshot file will likely be large.
Please note that your next reply will be private, visible only to you and me. You can paste the link to the file there. Rest assured that once the issue is resolved, I will delete the local site.
I now have a duplicate copy of your website using Duplicator. I will proceed to investigate this matter on our development site and will provide you with an update as soon as possible.