The JP Only is not intended to be an insult or discrimination. But on the receiving end due to the phrasing, it's quite easy to interpret it as such.

It's like if someone comes up to you in another language and you respond, "I only speak English". Or something like "Je ne parle pas Francais" (with the little cedilla on the c) or "No hablas Espanol" (with the necessary accents) (I don't know if any of these are correct typed out).