Add locale support: Unit of distance

[martinhpedersen]

We should support multiple locales to print distances in the user's preferred units of measurement, e.g. in the RMS list as requested. Metric vs imperial.

The request header Accept-Language and env vars like LANG could be a good source to provide a good default, but a config field is probably justified as well.

[martinhpedersen]

Nautical miles is also requested. Don't believe we can infer that from a HTTP header, so a config field is definitely needed.