How should a job title be translated?
There's no single right answer here. You should provide the information that you think is most salient in the given situation. In your case, you probably want to say something like "tenured research scientist" or "staff scientist," as those terms normally give the idea of permanence in a way that "research scientist" or "research associate" might not.

If you are writing a CV in English, on the other hand, you could write "Junior research staff (permanent position)," as that gets your title more accurately. In this circumstance, it's probably better to go with a literal translation (or as Federico Poloni suggests in a comment below, the job title in the original language) plus explanation, rather than try to find the "closest equivalent."