Stephanus and Marcus are being addressed in this case; you are saying "salvete" to Stephanus and Marcus. Most masculine words ending in -us (2nd declension) will get the ending -e in this situation. Names ending in -a don't change. (Salve, Livia!)
This is the vocative case, used for people being addressed.
However, words/names that end in -ius, regularly change that to -ī in the vocative, not -e.
Vergilius (nom.) - Vergilī (voc.)
So are you saying that it is considered incorrect to carry the vocative form over into English?
(I believe a similar thing happens in Russian - that is, names are declined for case. Although a bit of googling suggests that the vocative case has died out in modern Russian. Even so, would native speakers of Russian only use the nominative form of names when speaking another (case-less) language?)
Imagine that this is a text you need to translate into English:
Marcus vir est. Marcum amo. Marco rosam do. Salve, Marce!
(Marcus is a man. I love Marcus. I give Marcus a rose. Hi, Marcus!)
Marcus, Marcum, Marco, and Marce are all referring to the same person (Marcus). Using the different forms in English would only cause confusion, so no, we don't carry the vocative form over into English.
Vocative is no longer used in Russian, except for some very rare fixed phrases or for purely meme-purposes.
My knowledge of Russian is rudimentary. But I'm a native speaker of another Slavic language that has 7 cases. Trying to understand your question, but it's challenging. When speaking in another case-less language names are not being used in any other form except for nominative. It would not make a lot of sense to transfer vocative, especially to transfer only that one case.