Перевод - Финский-Английский - sinäpä söpö tyttö olet. miten sä suomea osaat?

Hi Bamsse

"How come you're speaking finnish?"

By "how come" ...do you mean "why"?
Being a question it should be
...are you..?
Also, Finnish requires capital letter.

The original text is written in slang.
To reflect this 'how come' is deliberately chosen instead of just 'why'.
Capital letter in Finnish is of course OK.

OK, then. I just wanted to be sure that it really meant "why".
What about the interrogative form?

Hi Bamsse, Hi Lilly

I was just wondering about the word "sweetiest", which seems somewhat strange to me, even for slang.

Maybe "sweetest" would be more "usual"

Bises
Tantine

it s "How do you speak finnish ?"(almost same as why can you speak finnish?)

The difficulty here is to convey the meaning of the "-pä" in sinäpä. Some other possiblities for you english-speaking people to consider what would be the best way to express the idea:
(How cute/sweet girl you are)
But you are a cute girl
What a cute girl you are
I don't know it the superlative form conveys the same message, might well be.

As for the second sentence it says how it is possible that you CAN speak Finnish, maybe:
(How come you speak Finnish?)
How did you learn Finnish?

I would define the original as spoken language, not exactly slang.

I think the translation should be: "You sure are a cute girl. How come you know Finnish?"

sweetest