My wife is currently involved in translation projects in the field of nursing in Serbia so I can give you an inside view on this:

This title is soon to be changed in the Serbian system, there will soon be no "visa medicinska sestra", but something like "strukovna sestra" - term not yet decided. In the meantime, the use of the term "professional nurse" is advised, while the actual diploma title would have to be descriptive: ("2 and a half year [or 3 year, this depends on which year she studied, it changed from 2.5 to 3 years a couple of years ago] post-secondary qualification").

I would NOT advise the use of "Senior Nurse", since the lowest- "ranking", youngest nurse on the ward can be a "visa medicinska sestra" but you would certainly not call her a Senior Nurse!

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Note added at 22 hrs (2007-04-18 22:12:49 GMT)
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If you are translating somebody's diploma, I would still keep the term RN, or PN, and add under it in brackets: a 2 and a half year post-secondary level programme, or 4 year high-school level programme. Definitely NOT junior nurse and senior nurse!!! C'mon, a "junior nurse" with, say, 30 years of experience? These are not diploma titles anyway, but rather job positions in the hospitals.

They are not graduate nurses either, as 2 and a half years of post-secondary education is not a graduation level.

If you are considering "registered", that should be fine for both, srednja and visa, as that is what they ARE by their qualifications and job description. Not legally though, as Serbia does not have the licensure system, but then you can actually argue - if there are no registered nurses, then there are no non-registered ones either, so RN is fine here.

One last comment. Whatever the choice is, it should be a translation into English, meaning that it should be clear enough for those in UK, Canada etc. and, like Vesna suggested, leave the validation in particular country to the legal experts.