Halo there I'm a native Cantonese speaker from Hong Kong. I don't think there's any formal dictionary for Cantonese but perhaps you could search for some useful Cantonese apps on Play Store (I've just searched and there's quite a lot of choices). I highly recommend you to listen to our Cantopop as well as our news on television. By doing this, you could learn more slang and informal spoken language. Cantonese is not that challenging as you think. There aren't lots of grammatical rules. I'm sure that you will definitely improve your Cantonese as long as you practice it more often. I hope this information will help. If you need any further information, please feel free to contact me ;)

Mandarin is better