The word makes a Russian smile, I've noticed :). Often add there "...sköter sjösjuka sjörövar..." to transfer it into a grin.
diminutives: Sylwunia, Sylweczka, Sylwusia or Sylwuś
Seems like diminutives formation is common in the Slavic languages. I would stick with the second. Sylweczka. :)
What about Sylwka?
I guess it would work in Czech. They tend to be lack of vowels. Prst, krst, krk. I am itching to add some vowelsto the one above. :)