If you are not using the dutch language you could also translate the dutch language to english and then set everyones language to dutch. i am not sure if it will work, but my guess is this will also change the locale and thus your decimal separator.
I think the fastest way is to do the following:
Empty all Dutch texts:
a) Go to menu Tools > Language operations b) Select all rows (modules etc). c) Choose 'Delete' as operation d) Choose 'Dutch, Netherlands' as language e) Click 'Apply'.
Update all users to use 'Dutch, Netherlands' as their language.
All texts will be English because if no Dutch text is available, English will be used as fallback. However, the localization of dates and numbers will be Dutch.
Note that if you add new widgets to your project, you will have to empty the Dutch texts again. The Modeler adds English US and Dutch texts by default to something like the New button.
7 years later still no better solution than highjacking a language?
There are several ways to do this:
It's possible to add some custom formatting for date and time in your project settings -> languages. This way you can create your custom format for date/time. You could file a request to do this also for custom formatting currencies.