Choosing a date from the Due Date picker returns an error

Summary

When editing an issue within JIRA and choosing a Due Date using the calendar picker, the user gets the error The Jira calendar doesn't currently support your language. Users experiencing this have their language set to English (US). It happens for all the users in the instance, and regardless of what browser or OS they are using.

Steps to Reproduce

1. Click Edit on any issue
2. Select a Due Date using the calendar picker
3. Click Update. It returns the error "The Jira calendar doesn't currently support your language"

Expected Results

The issue would be updated with the desired Due Date.

Actual Results

The error The Jira calendar doesn't currently support your language is thrown and the changes are not saved.

Workaround

Check your user profile settings and make sure the language is not -1 (see screenshot).