Make the 404 / 403 page setting "translatable"

Description of the need

Currently the 404 and 403 page setting isn't translatable, so all languages on a multilingual site get the same path. Even it that page is a translated node.

Proposed solution

We could either add per-language settings if the language module is installed, or we could follow the node translation if the page is a node and translations exist.

Alternatives that have been considered

• A layout (single path) that uses the translated node as "existing content"
• Contrib module

Both don't seem ideal, as the former requires some knowledge of the layout system, the latter requires people to know about the module.

Is there a Drupal or Backdrop contributed module that accomplishes this?

i18n does that for D7 (with bad UX), but that feature requires the (unported / unportable) Variables module for i18n_variables.

Additional information

This popped up in the i18n (contrib) issue queue.

See this comment for the steps to do it with a layout. We can't assume, a Backdrop newbie is able to do this.