Use a dropdown instead of rendering each flag. Although it doesn't look
as nice when there are only two or three options to choose from, it:
 * Works well with lots of translations.
 * Falls back gracefully when the relevant icon is not found.

It redirects the browser via JS as soon as a new language is chosen.

When JS is disabled, it falls back to showing a "Change" button, which
in concert with a `.htaccess` `mod_rewrite` rule, takes incoming
`lang=fr` query parameters and rewrites them to `/fr/...`

Icons are from the
https://github.com/googlei18n/region-flags/blob/gh-pages/COPYING
project, which subsequently took them from Wikipedia and confirmed they
are in the public domain.