You should be able to find what you need here :-
Setting up i5/OS with a national language version.
First, you need English installed along with Spanish. Then you can create a subsystem that defaults to English by setting the CHGSBSD SYSLIBLE() attribute to assign your English system library or an English application library by default. When you sign on, have your jobs routed to that subsystem.
If your applications are not coded for a language, you won’t have a choice for them. Only system displays will react.
Routing programs in a subsystem can also help assign libraries.