Unfortunately, localization has not been properly addressed, in my opinion, so we're not going to be able to use it. Yes, theoretically people should choose their locale in their browser settings, but most people do it on a site-by-site basis.
Though the date and time pickers which have been produced so far (outside of mobile phones maybe) are essentially garbage, in no small part for exactly the reason you suggest:
> Unfortunately, localization has not been properly addressed
which could even be extended to formatting in general. They also tend to look... bad. Not just "it's not a work of art" bad, but "this shit looks worse than most HTML/JS widgets created in the past 5 years and there is absolutely no hook anywhere to style them to match one's site"
Out of curiosity, why? I am always frustrated with Google (and others) for using my IP address to determine my locale setting, when I clearly have indicated en_US in the headers I send down to them. But I can see why they do if a lot of people purposefully don't set their locale correctly for whatever reason.
* Consistency 1. Most sites I spend my time on have English content. Same goes for programming, keywords are English, comments end up being English, etc.
* Consistency 2. Not every program I use has a (complete) Dutch version.
* Looks. Not everyone out there seems to take into account different languages when designing. Some English things cannot be said as shortly in Dutch without sounding silly (this works both ways). So pieces of the UI will look either overcrowded or overly empty. (Or the text sounds so odd because it got artificially shortened to fit)
* Convenience 1. When looking up an explanation/details about some programs you are using, the info will almost always be in English. It is easier to use the same language to find back what you need.
* Convenience 2. When reporting problems/bugs/finding out info about it, it is easier to use the English term used to find the info I need.
Those are the main things coming to mind. It's not like I've ever had real trouble understanding English anyway, so it's not a bother to put everything in English.
Keeping all that in mind, I do like my dates/times/... like I am used to them. Weeks start on Monday, times are in 24h clock, 16 Nov instead of Nov 16, 16/11/2012 instead of 11/16/2012, ...
LANG=es_ES LC_MESSAGES=en_US
(or GUI equivalent)?