You can make the Scala a lot faster by modifying your isPrime method to
def isPrime(n: Int, i: Int = 2): Boolean =
if (i == n) true
else if (n % i != 0) isPrime(n, i + 1)
else false
Not quite as concise but the program runs in 40% of the time!
We cut out the superfluous Range and anonymous Function objects, the Scala compiler recognizes the tail-recursion and turns it into a while-loop, which the JVM can turn into more or less optimal machine code, so it shouldn't be too far off the C version.
See also: How to optimize for-comprehensions and loops in Scala?
