Currently an entity's slug is has two customisation options: Slug directory and Slug item prefix which support variables like #T:

This still limits urls to a "flat" structure always having the same base slug directory. To create more meaningful urls, I would like to have extended url customisation with the support of an entity's field values. For this I would propose a syntax like :[field_name].
If for example I have a blog-post with a field named category, I would like to be able to use the value of this category in the url. I would be able to to this by entering :category as the slug item prefix, like so:

If I would now create a new blog-post naming it Highest buildings and selecting the category Architecture, this would result in the slug blog/architecture/highest-buildings.

This would also be a solution for i18n support in the front-end. In your entity's definition you could at a field for language. In the slug you could use :language/blog. Since this would be an entity property in the templates as well you can also use it to set the lang attribute in the HTML.