Hmm, yes. "Todella on" is meant to emphasise the "on", as in "English really, truly IS an important language". In Finnish "Englanti on todella tärkeä kieli" would then emphasise "tärkeä (kieli)" -> it's a REALLY IMPORTANT language.
Agree with the above. The answer should be "English is a really important language". Putting the "really" where it is indicates a cynical or argumentative tone that i suspect isn't present in the Finnish