[i18n] Support locale-specific search

[chalin]

For a good example, see, for example:

• https://glossary.cncf.io/fr/search/?q=m%C3%A9trique
• From https://github.com/cncf/glossary

Also fix (undo):

• /docs/faas/* /ja/docs/platforms/faas/:splat
• ...

Contributes to #4467.