Even using 4 languages is too many, e.g. Java and Scala for system building, Apache Groovy for build scripts and unit testing, and Javascript for web side as in

Even using 4 languages is too many, e.g. Java and Scala for system building, Apache Groovy for build scripts and unit testing, and Javascript for web side as in your example. Go with something like Kotlin, which compiles to JS, Android, and JVM bytecode, can be used for Gradle build scripts, and has builder-style syntax useful for test scripts. A single language is better than 3 or 4.