All four of these towns have excellent school systems. They are among the best in the state and that means something since Connecticut is so highly regarded in its public education system. For information on each system, check the links below.
Strategic School Profiles by District
Strategic School Profiles by School
IMHO, you can not go wrong with any of these town's schools. It is more a matter of the type of community you would feel best living in. Avon is probably the most upscale of these. Farmington is a bit more mixed as is Simsbury. Granby is a bit more rural in character but has some of the friendliest people I have met in the State. Each one is nice and would be a great place to raise kids.
I can not answer your question about languages offered but I would be surprised if one of these towns did not offer both. Here in Glastonbury they do and the towns you are looking at are comparable. Jay