Using Multiple Languages while developing a Spark application
An experienced developer will be able to pick up a new language and become productive fairly quickly.
I would only consider using the two languages together if:

The deadlines are too tight to allow for the developer to get up to speed,
The integration between the modules is quite limited (and you're confident that won't change) and
There is a clear deployment strategy.

I would suggest doing a small-scale test first to confirm the deployment and integration plans you have will work.
I'm working on a project with another person. My part of the project involves analytics with Spark's Machine Learning, while my teammate is using Spark Streaming to pipeline data from the source to the program and out to an interface.
I am planning to use Scala since it has the best support for Spark. However, my teammate does not have any experience with Scala, and would probably prefer to use Python.
Given that our parts of the program are doing two different things, would it be a good idea for us to have his Python script call my Scala executable? Or would using different languages raise complications later on?