Use browser's language preferences where possible

[hughbris]

This is a fancy nice-to-have. If we support more than one language, it would be optimal to look at the User Agent accept-language headers and see if we can accommodate those. Some thought required on the logic before commencing. Something like:

• get the UA language preferences
• see if they have translations here
• yes and already in $languages: move preferred languages to top in order
• yes and not in $languages: add supported preferences to head of list, retain current first/default
• no: do nothing

There are more complex scenarios than that!