build.gradle

static def createStringsFromJson(File inputFile, String outputFolderRootPath) {
    def json = new JsonSlurper().parseText(inputFile.text)
    def sw = new StringWriter()
    def xml = new groovy.xml.MarkupBuilder(sw)

    def keywords = ["abstract", "continue", "for", "new", "switch",
                    "default", "package", "synchronized",
                    "boolean", "do", "if", "private", "this",
                    "break", "double", "implements", "protected", "throw",
                    "byte", "else", "import", "public", "throws",
                    "case", "instanceof", "return", "transient",
                    "catch", "extends", "int", "short", "try",
                    "char", "final", "interface", "static", "void",
                    "class", "finally", "long", "volatile",
                    "float", "native", "super", "while"]

    xml.resources {
        json.each {
            k, v -> string(name: (k.toLowerCase() in keywords ? "str_" : "") + "${k.toLowerCase()}", "${v.replace("'", "\\'")}")
        }
    }

    def language = inputFile.name.substring(0, inputFile.name.lastIndexOf("."))
    def outputFolderName = "en" == language ? "values" : "values-${language}"
    def outputStringsFile = new File("${outputFolderRootPath}/${outputFolderName}/strings.xml")

    outputStringsFile.write(sw.toString())
}

task convertLocalizedJsonToStrings {
    def translationsDir = new File(translationsRepositoryLocalPath);
    translationsDir.eachFileRecurse(FileType.FILES) { file ->
        createStringsFromJson(file, "app/src/main/res")
    }
}

gradle.projectsEvaluated {
    preBuild.dependsOn('convertLocalizedJsonToStrings')
}