Hi Marlon
Check what is the Default Language set for Workflow user WF-BATCH (check in tcode WF-BATCH). Background step is executed by user WF-BATCH and the language in SY-LANGU can be picked up from there (not too sure).
Also, what is the Workflow Initiator's logon language (when the WF was triggered) Vs the Default language in SU01D for that user?
regards,
Modak