Everybody's going to say "Antigua" and they do have a lot of language schools there. Also, they have a lot of English speakers in both the local set and traveller set. You won't learn as fast.
Let me suggest San Andres. It is in the Peten region to the North of Guat. Easily accessed by a bus from Belize City. There is a school there called Eco Escuela that I've gone to twice. They use they make from the school to improve the town and area. There are very few other travellers in the town, though Flores is easily accessed by colectivo or lancha. It's right on the Lago de Peten Itza and you can swim there every day. They have homestays, should you prefer that, or you can stay across the water in a hotel in Flores. The townspeople are great. The life is relaxed and you are living in a Guat paradise. The town is safe and the school is right across the street from the police station. Tikal is not far away. There are banks in Santa Elena, across from Flores and just as close to San Andres, that take international Bank Cards. There is no downside there.
GLOT
D