I'm translating a Portuguese text into English and in the end there is a quotation in English, from a speech. I would like to know how to continue? Is it conventional to put the quotation in italics?
Quote it as you would quote any other part of speech, whatever the language, according to the style required by the publisher: usually quotations are indented and, sometimes, in italics, but check the specific style guide.
Then, you can add a note specifying that the text was originally in English, if you think that this piece of information is important for the reader or in the context.
The Chicago manual of style [1, §13.71] recommends:
Typographic style of foreign quotations. Quotations in a foreign language that are incorporated into an English text are normally treated like quotations in English, set in roman type and run in or set off as block quotations according to their length. [...]
So, at least one style guide recommends to not change the quotation style, whatever the original language. As I suggested above, I'd simply add a note stating that the original text was in English.
But given that that translation is part of your evaluation of a Translation course you're taking, if you are still in doubt, ask the instructor.
 The Chicago manual of style, 16th edition, The University of Chicago Press, Chicago, 2010.