Is there a Scala client library for Apache OpenWhisk?
The short answer is that there isn’t an Apache community supported Scala client. There is quite a bit to seed such an implementation from the Scala REST and CLI bindings defined by this interface https://github.com/apache/openwhisk/blob/fcbe9ca83829f2194b47f7c61a166396838c6a44/tests/src/test/scala/common/WskOperations.scala#L163 but it is not a standalone client.
If it’s something you’re interested in contributing to the project and community, you should reach out on the Apache project dev list as noted in one of the comments above.