Steps to Reproduce:

1. Set up system locale as Deutsch (Deutschland).
2. Administration > Global Settings > Time Tracking
3. Deactivate time tracking, and set Time Format to days. Then, activate time tracking.
   
4. Reindex (if necessary).
5. Create a new issue, with the Original Estimate set to 1d.
   
6. Once the issue is created, proceed to edit the issue.
7. Automatically, the value inside Original Estimate would be 1t.
   
8. If you try to save the issue, error messages would appear stating that The original estimate specified is not valid.
   
9. You would have to change the value back to 1d to be able to save the issue.

Workaround:

1. Administration > Global Settings > Time Tracking
2. Deactivate time tracking, and set Time Format to pretty. Then, activate time tracking.