hi, can i user guide for users that create survey on serbian english?

how?
on your site i see manuals but i don't know how translate them

The manual is a wiki, so in theory anyone can contribute. However, I am not sure if the language wouldn't need to be setup first, before you can start translating.

You might want to get in touch here: www.limesurvey.org/about-us/imprint

However, I think more important than the manual to be translated would actually be to get the Serbian translation of Limesurvey off the ground. It is currently only 56% translated:

translate.limesurvey.org/projects/limesurvey-3/

And there are actually two different versions for Serbian:

Serbian 56%
Serbian → Serbian (Latin script) 50%