starting a path alias with a langcode has weird behaviours

Problem/Motivation

If you start a path alias with a langcode, either by coincidence or because you think you need to include it, bad things happen.

Steps to reproduce

1. Set up a site to have two languages, say English and French
2. Create a node with both languages
3. Try to set the alias for the English translation to '/fr/yes-this-is-confusing'

The alias is NOT applied to the en translation, but instead gets put on the fr translation where it ends up showing as '/fr/fr/yes-this-is-confusing' (repeated prefix) in the form.

TODO: test what happens when trying to set that alias on the fr translation.

Proposed resolution

Either tell the user they can't use the langcode of an active language as a prefix, or handle it properly.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet