site.webmanifest needs localisation

[b-n]

The site.webmanifest is used for PWA and might help browsers decide some themes when browsing the site. PWAs were a thing, not anymore, but I hear it's good for them to be accurate.

Idea: repeat site.webmanifest into localised versions (e.g. en.site.webmanifest), then make sure the link in the header is referenced correct:
https://www.w3.org/TR/appmanifest/#using-a-link-element-to-link-to-a-manifest