Enhancement for translations: inherit root node slug from default language to non default languages

[ThiloWitt]

When adding a new local to a site and translating content into it it is easy to forget translating the root node and its slug into the new language. The root node does not show in the front end where translations can be done conveniently. Automatically inheriting the root node slug from the default language to non-default languages would solve the issue.

[Guite]

Note: see #594 for more information about the issue that is caused by a missing translated root slug.