I assume that you want to change an email subject (or title). To be really sure that you do everything correct, open the step in the workflow template like you probably already did (transaction SWDD). Open the control tab. Check the task ID (TS with 8 nunbers). Then open transaction PFTC, choose standard task from the drop down menu, and enter the task number (without TS). Now you can change the work item text (=subject). Before doing this, log in to the system with the language for which you want to maintain the text.
After changes, run SWU_OBUF, and start a NEW workflow to test your changes.
Regards,
Karri