SMILE, Haifeng Li’s Statistical Machine Intelligence and Learning Engine, includes a Scala API and relies on ND4J/ND4S for numerical computation.
Cortex - Twitter uses Scala for much of its big data stack.
MLlib is part of Apache Spark. While it includes several popular machine learning algorithms, it is not archtected to perform computation efficiently. For better architectures using Spark for machine learning, here is Deeplearning4j’s integration with Apache Spark for distributed neural net training.
MxNet has a Scala API.