Digitalia films contain two options:
1) Embedded subtitles: appear automatically without you doing anything
2) Optional subtitles: you can activate or deactivate them with the "CC" button that appears in the bottom bar of the viewer. Likewise, if there are several subtitle languages available, you can select the one you want from the drop-down that will be displayed.
If the "CC" button does not appear, it means that the movie does not have subtitles, or they are embedded. Please check the "Subtitles" field on the film's tab to find out if subtitles are available.