I want to get a query result by locale, but it appears that
LC_COLLATE cannot be changed.
This is an example of changing en_US.UTF-8(default) to ja_JP.utf8:
You should be able to create the collation with
CREATE COLLATION ja_JP (locale = 'ja_JP.utf8');
ALTER TABLE table_name ALTER COLUMN column_name TYPE VARCHAR COLLATE ja_JP;
Please refer to https://www.postgresql.org/docs/current/collation.html for more details.