Xcas has its own language which it uses by default, but you can also use either Python (with the option having the ^ character represent either exponentiation or the exclusive or operator) or the language used by Maple, MuPAD, or the TI89 calculator.
You can set which language Xcas uses in the CAS configuration screen (see Section 2.5.7). You can also set the language with the xcas_mode command.
The language you choose will be stored in the configuration file (see Section 2.5.10), and so can also be set there.