I think you can translate it as tortoise as well. If you check here: http://www.linguee.com/english-portuguese/?query=tortoise Even official translations refer "tortoise" to "tartaruga".
No. In my (British Commonwealth) kind of English, usually "tortoise" refers to the animal that lives on land, and turtle to the one that lives on water. But American and UK usage can differ too. Compare en.wikipedia.org/wiki/Tortoise and http://pt.wikipedia.org/wiki/Testudinidae and http://pt.wikipedia.org/wiki/Testudinata
I think it would be best not to use this animal as it seems to be one where the taxonomic conventions of both lanugages are a bit confused and not the same in each language either.
What do you mean "No"? Your Wikipedia page says the same as me: "sendo o termo tartaruga utilizado para todas as espécies de tartarugas." Here's an English source: http://dictionary.reference.com/browse/turtle?s=ts
But if you're being faithful in your translation to the original sentence, you would translate "turtle" as "tartaruga".
I'm sorry, I should have been more clear. In my kind of English, "tortoise" is not a type of "turtle". If anything, the other way around. See the English Wikipedia article "usage" section. The dictionary entry you link to seems like US usage to me. And here http://dictionary.reverso.net/english-portuguese/tortoise I get "tartaruga" as the result. So... all I can say is that this is a very confusing animal to use in a beginner's translation exercise!