Now, this is interesting. I've noticed that with every language I've studied or looked at, the same word seems to be used for both tortoise and turtle; either that or they use the words interchangeably but default to turtle. Only (British) English seems to distinguish between what are actually two entirely different species - that I've noticed.
Even Americans seem to use turtle to mean tortoise - something that had me baffled for a while in the past. Meanwhile, the Australians, on good contrary form, use tortoise to describe a freshwater turtle!
In case anyone else was ever curious about this; it seems that both words come from the same Latin root tortuca. The separate English words coming via Late Middle English tortu and turtuce; themselves from Old French turtue and Spanish tortuga.
Since many/most American words are derived from Late Middle English that explains that; but I'm not sure the Australians have any excuse! :-) :-)