Bringing back some old memories. Took a very similar trip about 6 years ago on my first bike (honda shadow 600) which I bought on ebay 3 months prior. learned how to ride in those 3 months and than took it down to Costa Rica.
I see you're looking for a cool hip place to learn Spanish. Guatajuato is a great place. Its a university town full of young people so the party seen is one of the best in Mexico. Barely any gringos so its pretty cheap. Also the town is absolutely stunning. Oaxaca is really cool but I think you'll get pretty bored. The ride from Oaxaca to Puerto Escondido is a great one however so if you need to get your party on it'll give you a excuse to ride some seriously technical twisties. There are 2 routes and one is in real bad shape so make sure that you travel on the right one. I believe its the one that is more north.
Another option is to wait and take the course in Antigua, Guatemala. Place stunning, weather spring like year round, great english schools, dope party seen and close to a international airport. There are quite a bit of travelers going through but they are generally of the shoestring, international, adventure type.
If you have any questions about places down to Costa Rica shoot me an email. I've travelled extensively all over Mexico and Central America so I know the place very well. However, stumbling upon gems on your own is half the fun so i understand if you never write. I'm also running mototours through Mexico
so if your spanish picks up maybe we can work together in the future.
Via con Dio amigo