In this context, "rich" is best translated as 豐富. However, you may want to put a context on what is meant by "rich."
Cantonese is a rich language.
廣東話是一種豐富的語言。
or, better
廣東話是一種表達方式多變的語言。 (Cantonese is a language that accommodates a great variety of expressions.)
Cantonese is a rich language.
廣東話是一種豐富的語言。
or, better
廣東話是一種表達方式多變的語言。 (Cantonese is a language that accommodates a great variety of expressions.)
