In the sentence:
Cтуденты, переводившие эту статью, говорят, что она очень трудная.
¿Is the meaning one of a perfect action of the type "Students, having translated this article,..." or is it more general and vague as in "Students translating this article..."
The imperfective aspect is about state, and the perfective is about state transition.
переводившие here means "the students who have been translating the article," i.e. all those who have ever been "in a state of translating the article." As long as the student spent at least some time translating the article, they would qualify for переводившие.
Its perfective counterpart, переведшие, means "those who have changed the state of the article," turning it from "untranslated" to "translated". Just spending time on the article is not enough.
This phrase describes all the students who have had a hand in translating the article.
Not sure if Russian is classified as having perfect aspect, but переводившие definitely has a sense of something that is in the past. You would need to use переводящие, if you wanted to stress that the translation was on-going.
But переводившие wouldn't necessarily mean that the translation has positively finished. Say, students, last time I was in contact with them, were translating the article. I don't know where they are with their translation now -- it may be finished or it may not. Might be they have given up on it altogether. I could still use переводившие -- my best knowledge of the situation is from the past and at the time they were definitely translating it.
There is also переведшие. This one is much closer to having a definitive connotation that something has positively occurred.
Студенты, переведшие эту статью, говорят, что она очень трудная.
This expression would most certainly mean that the students have finished translating the article. Finished as in not abandoned it halfway, but translated the whole article.
Переводившие can also signal prolonged action in the past, while переведшие has a feel of abruptness to it. This difference can be used as a stress. For example,
Студенты, переводившие статью 2 недели, говорят, что она очень трудная.
vs
Студенты, переведшие статью за день, говорят, что она простая.
In the former переводившие hints at prolonged action and, hence, some effort. In the latter переведшие implies it happened fast and was effortless.
The participle переводившие is a Past Tense imperfective participle. So, it is "The students who translated this article said it was very difficult" (if we observe the sequence of tenses in English; otherwise, and literally, "say it is very difficult").
