You can't have a subtitle track on by default. Something has to turn switch it on.

If it is a menu based project, then simply changed the properties for the button that leads to your video. Select the button in DVDA, go to the Properties window, and find the Action section. Change the Set Subtitle Stream setting to indicate the subtitle track that you want used.

Alternatively, you can add a Introduction Media to your project. Then double-click that item, find the End Action section of the Properties window, and change the Set Subtitle Stream setting. If you don't want anything to play automatically when the disc starts, then simply make a very short video of black nothingness and use that.

Rob