Make Intellij Idea plugin work with Kotlin files

I've created this very simple Intellij Idea plugin which folds some reference expressions. It works great for Java files but it doesn't work for Kotlin.

Here is the source: https://github.com/nodes-android/nstack-translation-folding. 
I will include here the important parts: 
plugin.xml

</idea-plugin>
    <depends>com.intellij.modules.all</depends>

    <application-components>
        <component>
            <implementation-class>com.nodes.folding.TranslationFoldingBuilder</implementation-class>
        </component>
    </application-components>

    <extensions defaultExtensionNs="com.intellij">
        <lang.foldingBuilder language="JAVA" implementationClass="com.nodes.folding.TranslationFoldingBuilder"/>
    </extensions>

</idea-plugin>

TranslationFoldingBuilder.kt

class TranslationFoldingBuilder : FoldingBuilderEx() {

    override fun buildFoldRegions(root: PsiElement, document: Document, quick: Boolean): Array<FoldingDescriptor> {
        if (root !is PsiJavaFile) {
            return FoldingDescriptor.EMPTY
        }

        val descriptors = ArrayList<FoldingDescriptor>()
        // Get all the reference expressions in this Java file
        val referenceExpressions = PsiTreeUtil.findChildrenOfType(root, PsiReferenceExpression::class.java)

        // Some logic

        return descriptors.toTypedArray()
    }
}

My problem is that for Kotlin files the buildFoldRegions() is not called at all.