For create multilingual website with our joomla website builder
you need do same as in Joomla Articles
1. Add yet one language in Joomla content language
2. Enable joomla languages plugins
3 at time add edit category - you will see combobox for set this category language, after you set language foe some category. You may in another category to which you set another language you may set "associate category" on other language.
4 In Instance manager, for instances - all the same only language and associate instance you may set in tab "settings" at instance edit time
But Please remember: you will not see "language select list" until you not did points 1 and 2
There is not request layout
That mean: you remove some layout or unpublish but it use or in menu or in some module.
Please return back what you remove. Or please turn off all modules one by one - so you will which module return this error