I've been using English as my system language, but Greek for regional formats. The thing is I want the date and time to appear in English too, while preserving the rest of my settings in the regional format settings (e.g. euro sign for the currency, metric system etc etc).
The reason is clearly aesthetic, although I assume that for some other languages (e.g. Arabic, etc) it should be also a practical reason since the date and time in terminal would not appear correct.