Allow pre-defined page slug translations

[dd32]

As a spin off of #3046 it'd be great if we can allow rosetta sites to translate page slugs.

For example, currently we've got /plugins/ and /themes/ (both are actual sites though, not pages) and we're about to add /about/* (Which are pages), these english slugs are not very welcoming when everything else on the site is translated.

The translation should occur through translate.wordpress.org, I'd suggest something like _x( 'about', 'Page Slug' ); or something along those lines.
Combine that with a cron-job which pulls from GlotPress and pushes changes to rosetta sites, a site_url filter to change the english versions to the translated ones, and some code to remember old page slugs.

[dd32]

and some code to remember old page slugs.

Hopefully ​#core4328 can land to handle this.

[obenland]

There seems to be a refreshed patch on the Core ticket

[dd32]

Assigning a priority - This is unfortunately a low priority task given the implications of the change.