"Yo hablé mucho en español."
Translation:I spoke a lot in Spanish.
De español would be incorrect in this case, prepositions don't translate literally.
Most of the time, you don't need the personal pronouns. In fact when you add them, it emphasizes that pronoun and gives a slightly different meaning. Hablé español. I spoke Spanish. Yo hablé español. (Look at me. I showed off my skills and) "I" spoke Spanish. Or I spoke Spanish (but others didn't) for examples. It would depend on context.
I don't think the English is grammatically correct. Certainly no native speaker would say that in English.