adding a second audio track in DVD Lab Pro is easy, creating a button to play it is kind of, depending on what you want.
2 possibilities:
1: select the audio without starting the movie.
you can use a link for that. It is under link option to set an audio track.
2: select the audio and starting the movie:
to do that you need a VM command. Go to connections on the upper right (project bar). right click on an empty space in the main bar: add --> add command object --> in vts domain
double click on the vm command object to open it
click on script
in dvd vm commands select SetSTN and manually write in brackets (audio=0) to select the first audio track or (audio=1) to select the 2nd audio track and so on. It will look like that: SetSTN (audio=0).
then click on ok and close. Set the movie as end link to the command object. Now you can link from your menu to the command object(s) so by clicking it the movie will automatically start with the selected audio.
Careful: once the second audio is selected this might stay active for other videos that are also on your DVD. This can result in no audio playing for the other videos. To avoid that you need another command object that sets the audio back to the first one. Set the end and menu link for the movie to this command. Set the end link of the command to the menu. This way it will always be set back until otherwise selected by the watcher.
Complicated, I know.