They are pretty primitive but are intelligent. I think they are the ones who gave the Dwarves in Skyrim their names, Dwemer (means dwarf in Giant). Which makes sense why they aren't actually dwarves but called dwarves because to a Giant, everything is small.
I also heard somewhere the Giants have a language recorded for the game but Bethesda cut it out or didnt add it in the games release or maybe its an upcoming DLC quest.
Not quite right, 'Dwemer' I think means 'Deep Ones,' and is the elven name for the dwarves. Dwarves is the human name for them, and is believed to have been originally coined by the giants.
As for giant's intelligence, they're shown to have at least some understanding of trade and even money. *Minor Spoiler*
Spoiler In the side quest 'a night to remember' you steal a goat from a farmer in Rorikstead and sell it to a giant, so that you have enough money to pay back Ysolda.