select CONVERT('À è Ë','WE8DEC','AL32UTF8' ) from dual;

The second parameter of CONVERT is the destination, so you're converting to WE8DEC, not from it!

Also note:

Oracle discourages the use of the CONVERT function in the current Oracle Database release. The return value of CONVERT has a character data type, so it should be either in the database character set or in the national character set, depending on the data type. Any dest_char_set that is not one of these two character sets is unsupported. The char argument and the source_char_set have the same requirements. Therefore, the only practical use of the function is to correct data that has been stored in a wrong character set.

https://docs.oracle.com/en/database/oracle/oracle-database/23/sqlrf/CONVERT.html

So the question is: why are you using CONVERT?

If it's because you have characters appearing incorrectly in your client, you need to set NLS_LANG (and possibly chcp on Windows) to display these correctly.