What happens when the original document has mistakes and is not properly written? The answer depends on how the translation is going to be used.
If the purpose of the translation is to understand the meaning of the source text and the source document is poorly written, our policy is to make sure the translation is in fact well written, even if we have to edit the translation to read well. When the translation team has to make an educated guess to achieve this, they will insert comments to alert the reader.
Translation is an art in the sense that the translator must communicate the intent and ideas of the original text by deconstructing and rebuilding the content of a document, basically creating a new document. It is a science because it must be done following language rules and accuracy benchmarks.