Japanese translation of Rust by Example is not built #124641
Description
Location
The language selector of Rust by Example has the link to Japanese translation, but the link target (https://doc.rust-lang.org/rust-by-example/ja/index.html) is not found.
Summary
The latest Rust by Example has Japanese translation resource based on mdbook-i18n-helpers.
But the current bootstrap process by ./x doc doesn't build the translation.
If I open a PR to add the translation build process, is it acceptable?