It's hard to say without knowing more about your activity, but you're likely looking at an Oracle driver issue. One thing to remember is that the regional settings are per user, so you'd want to make sure you checked the ones for the Workflow Server service user, not your own.

Oracle documentation suggests that support for Unicode characters is off by default and can be turned on in the data source settings. But again, this is just a guess because I don't know what driver you're using.

It's probably best that you open a support case so we can look into this issue.