I checked your site and found that you're using the "Max Mega Menu" plugin to add a Mega menu on the Menu items. However, I would like to inform you that we currently do not have any compatibility information regarding this specific plugin. But, while searching, I found that the Max Mega menu authority has published documentation on Translating Mega Menus created using the "Max Mega menu" plugin. Would you kindly try to translate the megamenu by following their documentation below?
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I hope it will help. If you want to use String Translation to translate the contents of the Megamenu, then the documentation we have on String Translation will help you the most. The Documentation covers almost everything you need to know about the String Translation.
If you check the sidebar of the documentation page, you will find more documentation related to String Translation. Kindly take a moment to explore those documents as well to get a better understanding of the String Translation feature of WPML.
I hope it will help. But, feel free to let me know if you need any further assistance or clarification in this matter. I will be happy to assist you.