"They are a lovely couple."
Translation:He ovat kaunis pari.
Because the subject can't be omitted in third person. I also think that "kaunis" is not the best possible translation for "lovely" in this case and it's perhaps even wrong, even though it is the suggested translation. "Kaunis pari" is more like "a beautiful couple". I would translate "a lovely couple" as "ihana pari".
We currently accept ihana pari. I would add that while that's the word to word translation, it's not an idiomatic translation per se and I would not use it in everyday language, because it sounds very, very silly. When Finns talk about kaunis pari, they do not usually mean that these two people are physically beautiful, but rather that they look well suited for one another, that they look "beautiful" when they are together. In English, that would be "lovely couple". :)
Same here. "Kaunis pari" meaning "lovely couple" sounds weird and kinda clumsy to me. I've never heard it used in that sense, but rather "ihana/suloinen/mukava pari".
Such a small nation, yet so many variations and personal preferences when it comes to language. Anyone trying to learn Finnish: you have my respect! :D