I found this problem for which I could not work out any obvious solution.
I installed the Sydney theme, adding the official Sydey-child theme, to manage some additional decoration.
Everything worked fine.
Then I installed polylang, to have the website in two languages. I followed the instruction, created/saved pages/menu/strings.
Home page, slider, menu all work fine.
The problem: when I am in the primary language all pages look fine, but when I switch to the second language, the Header Image and most of the template disappear.
I figured out that this happens as long as I have set the Header Image to “random”.
As soon as I select a single image for the Header Image, everything works fine.
As a matter of fact, it is not a primary fault on the website core, but it is much cooler to have that header image changing while the user keeps browsing the site.
Can you please help / suggest any solution?