Koha is used worldwide and so you need to make sure you set your localization preferences so that options throughout Koha appear properly for your location/language.
Localization/Internationalization System Preferences
dateformat : Decide how dates are displayed throughout Koha
opaclanguagesdisplay : Decide if patrons can choose what language the OPAC appears in
opaclanguages : Decide which languages the patrons can choose from
language : Decide which languages appear in the staff client
CalendarFirstDayOfWeek : Define your first day of the week