This mandate only applies to business use. I.e. if you advertise for, or label products, you must only use "Marmelade" for the strange british thing.

I don't think I have ever heard the word Konfitüre spoken in daily-life, other on occasions where someone explained why Marmelade is labeld Konfitüre in the supermarket. Maybe this is a bit regional and up north they use that word (?).

So if I was to translate a novel that features a jelly sandwhich I'd confidently translate it with Marmeladebrot.

Apart from that, lot's of dialects have their own term for jelly, like Swabian dialect has Gsälz, and Baden dialect has Schlecksl.