SCOTT@orcl12c> CREATE TABLE test_tab (id  NUMBER, test_col  VARCHAR2(60))
  2  /

Table created.

SCOTT@orcl12c> INSERT ALL
  2  INTO test_tab VALUES (1, 'très')
  3  INTO test_tab VALUES (2, 'tres')
  4  SELECT * FROM DUAL
  5  /

2 rows created.

SCOTT@orcl12c> BEGIN
  2    CTX_DDL.CREATE_PREFERENCE ('test_lex', 'BASIC_LEXER');
  3    CTX_DDL.SET_ATTRIBUTE ('test_lex', 'BASE_LETTER', 'YES');
  4  END;
  5  /

PL/SQL procedure successfully completed.

SCOTT@orcl12c> CREATE INDEX test_idx ON test_tab (test_col) INDEXTYPE IS CTXSYS.CONTEXT
  2  PARAMETERS ('LEXER test_lex')
  3  /

Index created.

SCOTT@orcl12c> SELECT token_text FROM dr$test_idx$i
  2  /

TOKEN_TEXT
----------------------------------------------------------------
TRES

1 row selected.

SCOTT@orcl12c> SELECT * FROM test_tab WHERE CONTAINS (test_col, 'très') > 0
  2  /

        ID TEST_COL
---------- ------------------------------------------------------------
         1 très
         2 tres

2 rows selected.

SCOTT@orcl12c> SELECT * FROM test_tab WHERE CONTAINS (test_col, 'tres') > 0
  2  /

        ID TEST_COL
---------- ------------------------------------------------------------
         1 très
         2 tres

2 rows selected.