There are still caveats in QuickTime 7.4.5, the gray transparent bar will show up even if there is no text. At second the language seems to be taken from the alternate track settings, it is not yet clear how to add multiple languages.
QuickTime Subtitles - a glance from the future
A new subtitle format is coming to QuickTime
If you want to be an early adopter, you can do it yourself. Annotation Edit will support this feature when it is official.
A ready made example is here.
Here are the steps:
1. Open a h254/aac QuickTime movie with Annotation Edit, create your subtitles and define the QuickTime layout.
2. Export as QuickTime reference movie from Annotation Edit .
3. Export this movie with QuickTime Pro as 3gp2, set audio and video to pass through, any text encoding is OK.
4. The 3gp export sets the text behind the movie, get it to front by setting the text layer to a low negative value. Save it.
As you see the format itself is not that new. What did change is it's adoption by QuickTime.
5. Now comes the arcane part: Apple offers a tool named Dumpster to view and modify the QuickTime movie structure. Open the movie with Dumpster and change the subtype of the handler description of the text track from text to sbtl. Save the movie.
6. When you now open the movie in QuickTime player you will see this option:
The information how to do this is partially taken from the Apple discussions forum.
Please leave feature requests there.