In Germany, the letter S is usually pronounced /z/ when between vowels or in the beginning of a word when followed by a vowel (
Sara,
Lisa). Otherwise it's pronounced /s/ (
Sven,
Esther).
Svetlana is quite common among people of russian origin and I've always heard it pronounced /s/.
In my experiance, germans are quite good at using the correct pronounciation of foreign names (apart from nasal pronunciations and the French J). I've met
Sami,
Sydney and
Soline and the S was pronounced /s/ by germans.
"zvitlana" would be quite a tongue twister for germans.