If you are texting someone using an Apple product, and you are on Wifi, and the message color (blue) indicates that it is being sent as an actual iMessage, then there is no data plan charge. Just like there is no charge for using FaceTime internationally.
Just make sure it is not being sent as an SMS message (you can always turn off Cellular temporarily when you do the messages to make absolutely certain that it will not attempt to send it as an SMS).
Best,
GB