I have made an app using Google Translates Voice/Speech-functionality in some features. Is it legal use it for commercial purposes?
You will have to read two licenses carefully: First, the licenses for your software development tools. Second, if this is for example an app for Android phones, you would have to read the license for end users of those phones.
If the functionality is part of the operating system that every phone owner receives, then it is most likely that they are free to use any application using this functionality. Apple's app store makes some differences between commercial and non-commercial use, but only concerning how many users can install or use an app (while paying once), not concerning what a legitimate user is allowed to do.
So the most important thing to check would be your software development licenses. Also, Google may claim that actual recordings produced by this software are derived works from their copyrighted materials, so I wouldn't include such recordings in the app.