[Cerrado] Main Menu - Language swithche issue - showing broken html code

Background of the issue:
I am trying to set up a language switcher in the main menu for a website using two languages: English and Spanish. The goal is to display only the name of the other language in its original language at the end of the menu, without dropdowns or side-to-side options. The issue can be seen on this page: enlace oculto.

Symptoms:
When I mouse over the language switcher in the main menu, it shows a piece of HTML code: "<Span C" instead of just the language name.

Questions:
How can I fix the HTML code issue appearing on mouse over in the language switcher?
What configuration changes are needed to display only the language name without errors?