Errata corrige: you can't see the translation in Chinese only if you are registered. If you are not logged in everything works fine.

Thank you Martina, now is fixed.

We was saving the User language in the database, but inside an ENUM column that was actually lacking support for Chinese:

DESCRIBE user;
...
user_language ENUM(`it_IT','en_US','fr_FR')
...

Fixed running this query in order to allow the database to support almost whatever future language:

ALTER TABLE MODIFY COLUMN user_language VARCHAR(5);