The Indefinite Articles são sempre traduzidos?

I would say that in Portuguese you could translate both ways, without loss of meaning. Since "um artigo escrito por Matthew" is the same (generally speaking) as "artigo escrito por Matthew."

There are contexts and contexts, though. And I was referring to the reasoning of the English Grammar, not the Portuguese one.
So, sometimes the use of "by Matthew McCartney" would do, and should do (as in novels/books reviews, for example, to mention who wrote the work).

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Since you wanted to know the Portuguese side of it. In colloquial Portuguese, one can leave out the definite article and it wouldn´t make much difference in sense.
Other case is the use of the article "o" before names of people (in the Northeast region one just says "João" and in Sao Paulo you would hear "O João" more often than not. And it would mean the same thing (in spoken Portuguese I mean, generally when someone is recalling some facts/telling a story).
It´s a bit of regionalisn in play. Nothing to worry about, just to adapt himself if living in one region or the other.
So, it´s eventually up to you and your experience, taking in account the local language "mores" and uses. Of course, Portuguese is generally a bit more flexible when it comes with articles/word worder, etc.

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