I want to execute inside APEX a complex query extracted from a EBS R11 concurrent, but this query has lots of functions like USERENV('LANG') and USERENV('CLIENT_INFO').
How can I set this info in order to execute this query?
I know the values of these parameters, but my problem is to set them
You need to initialize an apps context before you do anything EBS related. The most common place to do this is in the application VPD section (been renamed to Pre-Rendering PLSQL or something like that since APEX 4).
Use a call to fnd_global.apps_initialize. You need to pass the appropriate values for the user, responsibility, application etc.