Translation is an imperfect art. Sources cited in research may rely on translated works. What obligation does a historian have to disclose who translated the primary document?
To truly analyse an ancient Chinese text for example, must you have the ability to translate it yourself? How do historians deal with this need for a middle-man when reviewing primary documents?
In any situation, it's of course better to know the language and read the original document yourself. If you must use another's translation, it's expected that you cite the translator, often with something like (Vergil, Aeneid 1.45, trans. Smith). For many documents and studies, though, there's not much of a difference in utility. The information contained in actuarial documents, like censuses or business records, and standardized documents, like legal procedures and property titles, is much more important the actual text itself. If Bob Smith translated one column of a census as "Residents: 500, 600, 550, 700," odds are you won't get something different. Now if you are, in fact, working with the Aeneid, you get limited returns from someone else's translations. You can easily talk about events or themes or plot, especially if it's in the context of another topic of study or in comparison to other works. But an analysis of rhetorical devices or imagery or meter is rather pointless if you're not actually reading the Latin
In my studies, most documents are in Spanish with a local language like tossed in occasionally or written in parallel. My Spanish is excellent, but my Quechua is limited to a basic understanding of grammar/synax and vocabulary picked out from local town and archaeological site names. Thus, I'm comfortable with using colonial government records with Quechua names and words glossed. However, I'll never do anything with Quechua literature or poetry.
If you intend to be a professional historian and have credibility in close reading of texts, yes, you need to be able to read it in the original language. The only way around this is to hire an educated nonspecialist who is fluent and can translate in a verbally annotated manner for you--but that is time-consuming and costly, and you must credit that person in the finished product. For historians working in multiple African languages, it's not uncommon to have interpreters. But again, you must credit them and their work. To fail to acknowledge translation is to claim knowledge that is not your own.
The peril of not mastering the languages of your field is of course that you will miss things or misinterpret them, which (as /u/CommodoreCoCo points out) may not be too difficult for very simple data, but if you can't read the idioms in a complex letter that may be a smoking gun for your main thesis, you've got a real liability. Fortunately, research can sometimes be great on-the-job training--especially if you have a mentor or colleague willing to share some of their knowledge as you work. But true fluency may never come--I doubt I'll be reading poetry in seSotho anytime soon, either.
Understanding languages is important: the more you understand, the more information you have, and the more clues you can use to solve historical problems. That said, it's still possible to extract information from sources written in languages you don't speak well, or don't speak at all. It depends what kind of document you're looking at, and what kind of questions you're asking. I've examined e.g. census returns in languages I can't speak very well, and still extracted information from them.
Another example: I once did some research comparing encyclopedia entries. I looked up "Slav" to see which nations were recognized and which weren't, and discovered e.g. that "Macedonians" appear on lists of Slavic nations only after the Second World War. It says something about international public opinion toward Macedonia. Anyway, for this research I looked at Swedish and Turkish encyclopedias and recognized the ethnonyms without having any background in Swedish or Turkish.
As for translated sources ... scholars will often have face the choice of a translated version of a source or a version they can't understand at all. A scholar interested in e.g. Italian Communism can hardly take the attitude: "I can't read German, so I guess I won't bother reading any Marx." Yet the bias inherent in translations is difficult to overcome: by definition, the only scholars able to recognize and correct such problems are those able to read the original language version.
As for crediting translators ... well, sometimes translators names are not available, and other times the historian may be constrained by the house style of the journal or publisher. It's more important to credit translators of novels, poetry and belles-lettres than translators of more humdrum sources, e.g. government reports, legal documents, business contracts, and such-like.
I don't quite understand why "commodoreCoCo" insists it's rudeness or plagiarism not to cite a translator. I've cited Putin's speeches from English translations on the kremlin webpage, and there's no translator listed. Or, to give a more historical example, if you cite Lenin's "the State and Revolution" and your footnote says "Vladimir Lenin, State and Revolution, (city: publisher, year), page number" well, your footnote may not be complete since you haven't credited the translator ... consult the house style of the relevant journal or publisher ... but it's hardly plagiarism, since you've cited your source.