There is one more problem with localization of DatePicker. In render method of DatePickerBaseAppearance there is hardcoded width (line : sb.appendHtmlConstant("<div class=" + style.datePicker() + " style='width: 177px'>"), wich is not enaugh for october in polish language ("październik"), 185px will be enaugh. On the other hand there is no good idea to put the width in this place - maybe it should be moved to the DatePickerStyle.
So, I tried to inject a recompiled com.sencha.gxt.widget.core.client.DatePicker.class to my project with the upmentioned changes to it. According to GWT 2.4.0 i18n implementations return firstDayOfWeek taken from .properties file and substracted 1. That means that:
DateTimeConstantsImpl_en.properties defines firstDayOfWeek = 1. Via JAVA implementation will programmatically return 0. This makes