Hello again, I finished working in your website.
To be honest, I was not able to find the root of the issue, after several changes to the code I just learned that your website was not supporting internationalization at all, so it was not really the plugin’s fault, I went to the global settings page and changed the interface to French and it didn’t work, so I decided to download a fresh copy of WordPress and replaced the admin, includes and content directories, and now everything works.
I left the old directories in the server with a different name [1][2][3] in case that you want to compare the code between these folders and the ones that I installed. I reactivated all the plugins as well as the custom theme that you were using, none of them were involved in the problem that you were experiencing.
Due to limitations in the access to the server (no SSH for example) I couldn’t go into details of why the directories that you had before were blocking the translation of the interface. Please use a tool like Meld [4] to compare the content of these directories [1][2][3] with these other directories [5][6][7] considering that you have SSH access it will be easier for you to find out what was happening there.
Please visit all the Sucuri plugin’s pages and verify that everything loads correctly on your side, I tested here and everything looks fine. If anyone else stumble across a similar issue, please be sure to have “gettext” installed in your server and that both “wp-admin” and “wp-includes” contain the correct source code.
Let me know if you need more information.
[1] /public_html/__wp-admin
[2] /public_html/__wp-content
[3] /public_html/__wp-includes
[4] https://meldmerge.org/
[5] /public_html/wp-admin
[6] /public_html/wp-content
[7] /public_html/wp-includes