Userenv is a pre-build context that describes the current session.
You can query the context Userenv with two functions.
SELECT SYS_CONTEXT('USERENV','SESSIONID') FROM DUAL;
SELECT USERENV('SESSIONID') FROM DUAL;
You can see all the SYS_CONTEXT parameters in the SQL documentation. Predefined Parameters of Namespace USERENV
USERENV('LANGUAGE')
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AMERICAN_AMERICA.WE8ISO8859P1
It gives the session's <language>_<territory> but the DATABASE character set, so the value returned is not the client's NLS_LANG setting!