The CSV Importer currently uses the Default Locale to map strings into dates. But then uses the User Locale to import the date into the issue.
It causes this error
com.atlassian.jira.issue.customfields.impl.CustomFieldValidationException: Invalid date format. Expected d-MMM-yy
> at com.atlassian.jira.issue.customfields.converters.DatePickerConverter.getTimestamp(DatePickerConverter.java:40)
> at com.atlassian.jira.issue.customfields.impl.DateCFType.getSingularObjectFromString(DateCFType.java:45)
> at com.atlassian.jira.imports.importer.impl.DefaultJiraDataImporter.importIssues(DefaultJiraDataImporter.java:428)
> at com.atlassian.jira.imports.importer.impl.DefaultJiraDataImporter.doImport(DefaultJiraDataImporter.java:86)
> at com.atlassian.jira.imports.importer.impl.ImporterThread.run(ImporterThread.java:16)