While #4 is the best single option (and the one I suggested the last time you asked 😉 ), in an ideal world, I'd plump for a combination of #4, with a banner offering to do #3, and (#6) a reminder that the user may also (perhaps later) create the article/ add it to article requests. After all, Wikipedia needs more editors!

Any or all of these could still be done under a 404 error code, of course (as en.wikipedia currently does for not-found articles).

I appreciate that option #6 might mean crowd-sourcing a set of translations; Google Translate may suffice in the interim.

Please let me know if I (a non-coder, alas) can help with any of this.