The placement of 'really' changes the emphasis and therefore the meaning - it's explained in the tips and notes for this section, about half-way down the page: https://www.duolingo.com/skill/fi/language_1/tips-and-notes
Yes, it was explained that by saying, "French really is a beautiful language," we are saying that it is beautiful rather than some other adjective... and when we say "French is really a beautiful language," or *"French is a really beautiful language," we are emphasizing the extraordinary beauty of it.
I don't entirely disagree. But we are talking about English semantics. As a native speaker and teacher of ESL, I would say that it's simply much more likely that we would start the sentence with either of the two variants that start with "French is." But neither are accepted. I have reported it. When they ask me to translate from English to Finnish I will use the syntax they suggest.
Which variety of English do you speak? I'm a native British English speaker, also an ESL teacher, and 'French really is a beautiful language' vs. 'French is a really beautiful language' are two equally valid constructions, carrying different emphasis. It seems that Finnish allows for the same distinction with the placement of 'todella'.