Does the timer work even if I leave the application?  For example, can I take a call or turn on music and have it still run in the background?

Yes.  The state of the timer will be saved so that when you return, it will continue to track your contractions as though you have never left the application.  To the user, it will appear as though it is running in the background at all times.  You can exit to turn on music, take a phone call, play a game etc. and the tracker will still work. 
However, please note if you are in another app and an alert occurs, it will not be shown.  You will only see the alert once you return to the Contraction Tracker.  This is one of the limitations of the iPhone SDK.

Will the Contraction Tracker be made available in other languages?

We have just submitted a translated version in French.  It should be available for all French users in the beginning of August.

How are the different phases during the 1st stage of labor defined?

The following are the expected values that are used.  Note, you can also see what sort of values are being stored when you auto populate the program by shaking the device under settings and choosing the option "Reset History & Populate."
Duration: 0-40 seconds
Interval: 30-60 minutes
Early Phase
Duration: 20-45 seconds
Interval: 5-30 minutes
Active Phase
Duration: 45-60 seconds
Interval: 3-5 minutes
Transition Phase
Duration: 60-90 seconds
Interval: 1-3 minutes

The Contraction Tracker USA is the same as the original Contraction Tracker, except for one change. But what happened to all the previous versions ratings and reviews?

Unfortunately since we removed the original application and replaced it with a separate new version, we lost all our ratings and reviews. The decision was made to offer a separate application instead of updating the original version because we didn't want the change to affect those who had already purchased it in the US. This also allowed us to continue selling the original application to the rest of the world. Knowing that we would lose all of our US ratings and reviews, we made sure we took a screenshot of the iTunes page before our new application was submitted.

For those who are interested, here is the screenshot showing the ratings and reviews of our Contraction Tracker:

iTunes Ratings and Reviews of the Contraction Tracker before we replaced it with Contraction Tracker USA