Broken link checkers should fallback nicely to English if need be

[peterbe]

See #3053 which this issue can be blocked by.
If there's a pure HTML link like:

<a href="/zh-CN/docs/Web/CSS/new-property"><code>new-property</code></a>

but that doesn't exist in zh-CN yet, we have a chance to fall back to en-US.
The Broken link checker always runs. If set, it also spits out a flaw. But every href value is checked.
And in the builder, we do have access to the translationsOf Map object so it's cheap to look up something. Like:

if (!exists('/zh-cn/docs/web/css/new-property')) {
  // But there's hope in English!
  if (exists('/en-us/docs/web/css/new-property')) {
    // Rewrite the <a href> and possibly add a tooltip or a extra class name
    ...
  }
}