Hi Eunice,
There is an article explaining how to fix it, I hope it helps!
https://confluence.atlassian.com/display/CLOUDKB/JIRA+Date+Picker+Indicates+JIRA+Calendar+Does+Not+Currently+Support+Your+Language
Resolution
- Log into the instance with the affected user and view their Profile Settings (Click on the User's Profile Icon in the Upper Right > Profile)
- Click on the Edit Icon () next to Preferences
- In this window, re-select your language from the Language Dropdown and click Update
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