OP i can understand why you would be a bit peeved. While i totally agree that it is brilliant encouraging children to speak another language, at such a young age it should be up to the parents. However, it would be quite strange to teach a child another language that the parents dont speak, or never use themselves.
In a way and it depends on the dynamics, i would find it quite rude of the grandparents as i would feel as though it is pushing the child into something because it is their decision, same with things such as religion and opinions and beliefs etc. Parents should always be consulted IMHO.