USERLOCALE() returns the locale designation provided by the user's device.
A locale a standard way to convey a preferred set of communication norms and conventions covering a wide range of factors, including: address format, currency, language, numeral system, punctuation, sorting, units of measurement, and many others.
In the United States, the common locale designation is en-US, indicating a preference for US-style English and US-centric conventions.
Locale designations are controlled by the user's device; USERLOCALE() merely makes the device-supplied value available to the app.
A Text value: the user's locale designation.