Support Accept-Language to set LC_ settings

[wolfgangwalther]

Now that we have timezone support via header, it would be cool to add support to change various LC_ settings via header, too. LC_MESSAGES is superuser-only and could mess up error message parsing by us on the server or on the client, too, so we shouldn't change it. But the other 3, LC_MONETARY, LC_NUMERIC, LC_TIME seem useful.

The first to come to mind is Accept-Language - but I'm not exactly sure how the "content negotiation" part of this would look like. If a language was accepted here, it would probably mean to return the actual content in that language as well - which is well beyond just setting some LC_ settings.

An alternative header would be another Prefer, of course. This could be made flexible enough to specifcy lc_monetary, lc_numeric and lc_time separately.