You are describing how you speak; "speak" is a verb. Therefore, you need an adverb—in this case, "badly." If you were describing your English ability, then you could say, "My English is bad" ("bad" is an adjective describing your English ability, which is a noun).

I think it must be "my English is not good" or "i am not very good in spoken English". http://englishlanguagetrainingcourse.wordpress.com/2012/09/29/english/

In the American dialect of English, the phrases "I did BAD on the test" or "I speak English BAD" are gaining acceptance amongst native speakers (usually amongst adolescents, although I have also heard American adults using adjectives in place of adverbs)   Since I am Australian, I consider the correct variants to be "I did poorly on the test" or "I speak English badly". Often Americans say to me — you speak English very good! Now you will know about this phenomenom — and you will not be confused when you talk to an English speaker, and they say "I did BAD..." — you will know what is correct )) All the best to you!

very bad