How to deal with the is_* conditional tags?

Off the top of my head:
- `is_front_page` – e.g. should `is_front_page` return true on `http://example.com/de/`?
- `is_home` – similar issues to `is_front_page`
- `is_page` – can we make this return true for post_types shadowing the `page` post_type, e.g. `page_de` and similar?
- `is_attachment`
- `is_category` and `is_tag` – how does this cope when on the archive page for a taxonomy which is shadowing `category`, e.g. `category_de` and similar
- `is_tax` – similar problems to the `is_tag`
- `is_page`
- `is_singular` – if you use `is_singular( 'post_type' )` what should happen when you're on a shadow of that post_type?

Thoughts?

Review the conditional tags listed in the issue and how WordPress handles front pages, home pages, pages, attachments, taxonomies, and singular post types. Agree on the intended behavior for language-shadowed URLs, post types, and taxonomies; done means the project has a settled, documented specification before implementation begins.

Written by the indexing model from the issue text.