The Subject picklist value on Task/Event records does not update to reflect the user’s selected language for existing records, even when translations are configured via Translation Workbench.

This behavior is working as designed.

The Subject field on the Task/Event object is a global picklist that supports multi-language translation via Translation Workbench.
Translations apply only at the UI-rendering level, and not to stored values. The picklist value stored in the database remains in the original value selected at the time of record creation.
The translated labels are resolved at runtime during record creation or editing based on the user's current language context.
However, for existing records, Salesforce displays the stored API value or label at the time the record was saved, without dynamically updating it to the current language preference.
This design ensures data integrity and consistent historical audit trails, particularly in multilingual orgs.