Using kcadm.sh to import localization file

Getting advice
1

Currently, i can create realm & user of realm via kcadm.sh. but I can’t import localization for my realm.
i’ve try:

/opt/jboss/keycloak/bin/kcadm.sh create realms/demo-realm/localization/ja -f demo-ja.json -h content-type=multipart/form-data

And server response

org.keycloak.client.admin.cli.util.HttpResponseException: null [unknown_error]
        at org.keycloak.client.admin.cli.util.HeadersBodyStatus.checkSuccess(HeadersBodyStatus.java:61)
        at org.keycloak.client.admin.cli.util.HttpUtil.checkSuccess(HttpUtil.java:351)
        at org.keycloak.client.admin.cli.commands.AbstractRequestCmd.process(AbstractRequestCmd.java:363)
        at org.keycloak.client.admin.cli.commands.AbstractRequestCmd.execute(AbstractRequestCmd.java:126)
        at org.jboss.aesh.console.command.container.DefaultCommandContainer.executeCommand(DefaultCommandContainer.java:63)
        at org.jboss.aesh.console.command.container.DefaultCommandContainer.executeCommand(DefaultCommandContainer.java:48)
        at org.keycloak.client.admin.cli.aesh.AeshConsoleCallbackImpl.execute(AeshConsoleCallbackImpl.java:54)
        at org.jboss.aesh.console.AeshProcess.run(AeshProcess.java:53)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
        at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.lang.RuntimeException: {"error":"unknown_error"}

Did you find out the solution?
Thanks.