Add i18n fallback and outdated warning

[mmmray]

See #529 for some discussion

and if the page is deleted:

the value of fallbacks is a bit questionable if the sidebar may already be incomplete.

I found that the devserver has to be restarted to show fallback pages after they have been deleted.

This relies on a fork of a vuepress plugin: https://github.com/mmmray/vuepress-plugin-i18n/tree/bump-vuepress

I only bumped some dependencies to make it work (vuepress and ecosystem)

the fork can be transferred to the xtls repo if this functionality is desired.

[yuhan6665]

looks good to me! We can definitely give it a try

[mmmray]

the plugin fork is moved to xtls org, let's try it out then