I know that in Hebrew it is written שי'.
How should I write it in English letters in short so that people will understand it?
Maybe she' or shi'?
I recommend "(may he live!)". It's unambiguous, unlike an abbreviation, which may stand for any number of things, and unlike, especially, a transliterated abbreviation, which may represent different Hebrew abbreviations to people with different Hebrew dialects. It's also more accessible to those who don't know Hebrew, or who don't know it well. And the parentheses set it off from the running text, so that, even though it's a slightly long phrase, the reader can treat it as an aside and it doesn't break up the flow of the surrounding text.