I just made my first translation here a few days ago. At the time, I noticed that some types of relevant reference/background information about the project was missing - information which would have made my work somewhat easier but above all helped to ensure that the end result meets the client's expectations.

I assume clients need to post some form or other when they post jobs. I would propose that this form has a few extra fields to encourage clients to submit relevant reference/background information (unless this is already the case).

Here are some types of information that often is very helpful to get:

(A) Special terms which should not be translated, e.g. names of companies and products, trademarks, etc.

(B) Other special terms which require special instructions along some lines or other.

(C) Special abbreviations used in the document, together with their meanings and instructions for how to handle them.

(D) Special machine-related symbols used in the document and how they should be handled (e.g. in case the document contains symbols or special bracketed expressions to be replaced by context-related strings, etc.)

(E) A brief, general statement of what the document is about (e.g. "This is about an XXX product used by YYY companies to ZZZ...")

(F) A brief, general statement about the intended audience of the translation. (e.g. "This is an internal report to be presented to the board.")

(G) A general statement about how to handle terms referring to graphical user interfaces (GUI).
PLUS an explicit list of any such terms that the client may be aware of, possibly together with specific instructions in each case.
For instance, in the case of a source sentence such as "Next press the Submit button", I would need to know whether the submit button is still entitled "Submit" on the GUI used by the readers of the translation (or whether they will be using a translated GUI).