Yes, LANGOUSTES is a Scrabble word!

Letter values in Scrabble US: L 1, A 1, N 1, G 2, O 1, U 1, S 1, T 1, E 1, S 1.

Is it a valid word?

LANGOUSTES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

warm-water lobsters without claws; those from Australia and South Africa usually marketed as frozen tails; caught also in Florida and California
large edible marine crustacean having a spiny carapace but lacking the large pincers of true lobsters

Also: spiny lobster, rock lobster, sea crawfish.