Simple address may fail for fallback language

Description

In some cases, where a fallback language is defined, a simple address might return a 404, but it should return a 200.

Steps to reproduce on Alloy:

1. Under 'Manage Website languages', enable 'en-GB' in admin mode.
2. In edit mode for start page, set 'en-GB' as the available language, and set fallback from 'en-GB' to 'en'.
3. In web.config, configure the <globalization> element with culture="en-GB" uiCulture="en-GB".
4. Create a page in 'en' with a simple address, e.g. 'simple', publish page.
5. In view mode, browse to '/en-GB/simple' .
6. In view mode, browse '/simple' .

Expected result:
200 responses for steps 5 and 6.

Actual result:
404 responses for steps 5 and 6.