You have two options.
In the File > Form Properties > Defaults tab, you can set the locale for the whole form. One option is to specify the user system locale.
If you want to do it on an object-by-object basis, you can specify the locale in the patterns dialog:
Note that in the patterns dialog, "default locale" refers to the setting you have in the File > Form Properties > Defaults tab (see above).
A word of caution, if the user's version of Acrobat/Reader or their OS is set to a different locale, then you may get different results.
You can also change the locale at runtime. See here: http://assure.ly/hVcqlj.
Hope that helps,