In SAP help, the determination of language code / locale is explained.
For example, when I access CRM application “My Appointment” with English language specified in url:
Https://:/sap/bc/ui5_ui5/ui2/ushell/shells/abap/FioriLaunchpad.html?sap-client=001&sap-language=EN#Appointment-myAppointments&/newappointment/20151014/ZXXX/true
The DatePicker looks like below:
