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That being said, we are working on the alternatives at the moment, because offering something unstable and unreliable to our users is the last thing we want to do.
We want to have the following hierarchy: Epic > Feature > Story > Sub-Task. What should we do?
There can't be the "Epic" issue type if you already have it translated ("Epic" renamed to "Feature"), because the issue type is still "Epic", even if displayed differently on the UI.
You can have such hierarchy if you create a new issue type with some other name like "Portfolio Epic" or "Epic1" or anything else rather than exactly "Epic". Otherwise there will be conflicts in Jira as it will consider them as duplicates.