The translation is transmitting the meaning of the text of the source language to the target language while retaining the meaning.
Translators always risk inappropriate use of idioms from the source language in the translation to the target language. But on the other hand such transfusions can bring useful calque, enriching the target language. Actually translators significantly helped formation of the languages.
Accuracy and transparency are twin ideals in translation, which often contradict each other. Accuracy is the extent to which a translation exactly transfers the meaning of the source text without distortion. Transparency is the degree to which the translation seems to the speaker of target language as originally written in this language and meets its grammar, syntax and speech. Translation that meets the first criterion is called "accurate" and the translation that meets the second - "idiomatic". The two qualities are not necessarily mutually exclusive.
Criteria for assessing the accuracy of the translation varies depending on the subject, type and use of the text, its literary qualities, social or its historical context, etc.
The criteria for assessing the transparency of the translation look simple: a non idiomatic translation sounds wrong, and in extreme cases the translations "word for word" generated by many machine translation systems are often nonsense. However, in certain contexts a translator may consciously wish to make a literal translation. Translations of literary, religious or historic texts often adhere as closely as possible to the source text, stretching the boundaries of the target language to get non idiomatic text. A translator can accept expressions from the source language to provide "local color".
The issue of accuracy against transparency is also formulated in terms of formal equivalence and functional equivalence. Formal equivalence corresponds to the literal translation and dynamic – to the transmission with own words. Functional equivalence transmitted the main thoughts expressed in the source text - if necessary at the expense of literalness and word order. Contrariwise, formal equivalence (sought by the literal translation), is trying to forward the text literally or "word for word" if necessary at the expense of the natural characteristics of the target language.
However, there is no clear boundary between the functional and formal equivalence, on the contrary - they represent a spectrum of translation approaches. Each of them is used in various positions and in different contexts of the same interpreter as well as at different points within the same text - sometimes simultaneously.
In all cases, the competent translation implies judicious combination of formal and functional equivalence.