The sentences "French is a really beautiful language" and "French really is a beautiful language" don't mean the same thing even though they are close. The same thing is true for the sentences "Ranska on todella kaunis kieli" and "Ranska todella on kaunis kieli". That's why "French really is a beautiful language" is the only right answer.
They aren't the same. In "French is a really beautiful language," the word "really" modifies beautiful to magnify it. Compare "It's beautiful" to "It's really beautiful." The second sentence is emphasizes more beauty than the first.
Now look at "French really is a beautiful language." We aren't modifying beautiful, we are modifying "is." How does this work? Maybe I'm unsure whether French is beautiful, but then I start learning it and decide, oh yes, "French really IS a beautiful language. Is French beautiful? It is! It really is!"
See how they are different? The "really" is modifying different things.