Rollup merge of #124731 - dalance:add_translation_support, r=ehuss

Add translation support by mdbook-i18n-helpers to bootstrap

This PR add translation support by [mdbook-i18n-helpers](https://github.com/google/mdbook-i18n-helpers) to bootstrap.
This is draft PR because there is the dependency to my forked mdbook-i18n-helpers.
If this PR is acceptable, I'll send a PR to the original mdbook-i18n-helpers and remove draft after changing the dependency to the original.

Closes #124641

Author: matthiaskrgr