Add locale switcher and store locale in a cookie

[HaraldKi]

The locale in a browser seems to be a bit of a guess and is hard to change for the user at least in firefox. Therefore we should provide a simple locale selector on the UI and store the result in a cookie.

The locale is currently used for thousands separator. Need to decide whether I want to use it for the date too.

If nothing is selected, we won't force anything into a cookie but just go along with whatever HttpServletRequest.getLocale() spits out.