我不要說中文= I do not want .....
我不知道(怎么)說中文=I do not know how to speak chinese
我不知道中文=I do not know chinese
我不能說中文= I can not speak chinese
我不可以說中文=I am not able to speak chinese
If you want tell somboday you don't know speak chinese, usually you can say'我不会说中文'