sqlplus /nolog

SQL*Plus: Release 18.0.0.0.0 - Production on Sun Mar 24 23:38:27 2019
Version 18.3.0.0.0

Copyright (c) 1982, 2018, Oracle.  All rights reserved.

SQL> connect SYS@PMJWE8 as SYSDBA;

Enter password:
Connected.

SQL> shutdown immediate;

Pluggable Database closed.

SQL> startup restrict;

Pluggable Database opened.

SQL> alter database character set internal_use WE8ISO8859P15;

Database altered.

SQL> alter database national character set internal_use AL16UTF16;

Database altered.

SQL> alter system disable restricted session;

System altered.

SQL> select * from nls_database_parameters where parameter like '%SET%';

PARAMETER                 VALUE
------------------------- -------------------------
NLS_NCHAR_CHARACTERSET    AL16UTF16
NLS_CHARACTERSET          WE8ISO8859P15