Jean-Luc's reply has emboldened me to present a couple of observations on this issue. But first, a clarification. In American English the word "brackets" refers to square brackets [like this], while the word "parentheses" refers to round brackets (like this). From some things I have read by British writers, this may not be the usage in British English.
Many times I have noticed something particularly interesting to me in an English translation of a Sanskrit text, and have then looked it up to find the Sanskrit behind it. Many times the Sanskrit is not there; the interesting English portion was silently supplied by the translator. The problem here, for me, is that things get attributed to a Sanskrit writer that are in fact by an English-language translator. So brackets are very helpful to me, at least in technical writings (obviously excluding such writings as novels).
This pertains to the issue of accuracy, which can be more important than readability. The large-scale translation of the Buddhist scriptures from Sanskrit, first into Chinese, and later into Tibetan, is probably the largest example known to history from which the effects of literary versus literal translations can be studied. The Chinese translations were literary, while the Tibetan translations were literal, so literally accurate that brackets were not required. In general, nothing was added that was not in the Sanskrit. The Tibetan translations were made literally by early royal decree, the idea being that the Buddha's words were too sacred to risk interpretation by translators. The resulting translations were not readily comprehensible to the people, as Geshe Lozang Jamspal assured me. The canonical translations were usually studied in Tibet by way of later commentaries on them written in native Tibetan.
In brief, Buddhism did not flourish in China, at least partly due to confusion of the meaning of the Buddhist texts resulting from their more literary translations, and contradictions between the different Chinese translations of the same Sanskrit text. By contrast, Buddhism flourished in Tibet, at least partly due to the consistency of the literally accurate translations of the Buddhist texts, with their standardized translation terms. The various Buddhist schools could arise in Tibet, with their various interpretations of the Buddhist texts, because they all started from the consistently same basis. The interpretations came later; they were not built in to the translations of the core texts by the translators.
No one in Tibet wondered, for example, whether a Sanskrit core text spoke of dhyāna or samādhi, because
dhyāna
was consistently translated as bsam gtan, while
samādhi
was consistently translated as ting nge 'dzin. The Tibetans did not have to contend with "meditation" or "concentration" or "meditative absorption" or "meditative stabilization" used variously for these two words in translations, like we have in English translations today, and apparently like what occurred in the Chinese translations. Since we do not have standardized translation terms, adding the Sanskrit word in parentheses is something I find helpful. Nor did the Tibetans have to wonder whether a word or phrase was added to the translation by the translator. When words or phrases are added, as I believe is often necessary in translations of terse Sanskrit into English, I find brackets to be helpful.
Best regards,
David Reigle
Colorado, U.S.A.