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."

Literal translation may be very misleading. For example, part time lecturers in Spain are called "professor associat" in Catalan or "profesor asociado" in Spanish. Literal translation of that title as "associate professor" would be very uninformative, since associate professors in other countries are full time senior faculty. To make things worse, protocol rules in the University of Barcelona used to state that in formal events the title "professor" should be used for teaching staff members who are not doctors, just making "professor" a lower title than "doctor", which is just the opposite of the usage in English speaking countries.

Therefore, I would not use literal translations. If I would need to be very precise about my role (for example, in a CV) I would give my official title name in its original language and a short explanation of what it means, for example with the rough equivalent in the destination language. However, in most situations we don't need to bother the audience with such details. If we are going to give a talk on any subject, the audience is supposed to be interested on what we can tell them about the subject, not about what kind of position we have, and saying which institution or department are we affiliated with may be enough.