There are many ways to replace the pronoun "it" depending on the syntax.
it=ele, ela, o, a, lhe, o (generic demonstrative pronoun and not personal pronoun).
In this case is "Diga-o em português" (demonstrative pronoun) = "Diga isso em português"
generic "o" (always masculine) may replace a word, a noun predicative, a sentence, or a text.