"Join the fun, learn spanish" would be: ' Únete a la diversión, aprende español'
You can say "Es divertido aprender otros idiomas".
unir
¿Puedo unirme? would be the best way to say "Can I join?" in Spanish.
I have fun with you: Me divierto contigo.
It will be fun - será diversión
Te has disfrutado = Did you have fun
french.... or spanish but i mostly say french so it can help you later on in life and it also fun and hard to learn
divertir
You could say "Pues, aprendalo."
gracias
aprender
If Mexican is just Spanish then have fun! is : ¡diviértase!