The word 'satan' is being used and heard as if it were a name. How could it be rendered in a way not distracting from its actual meaning of opposition and enmity?
In the Tanakh the concept of a "satan" exists, but it is not a personification of evil and there's no particular reason to believe there's even just one for all time. The word "satan" is a job description. The best way to render the Hebrew הַשָּׂטָן is probably literally: "the satan", lowercase 's', with definite article (the הַ). It would be misleading to render this "Satan", formatted as a proper name. An alternative is to translate the job description; this translation renders it as "the Adversary". (I don't know why they use a capital 'A'.)
Christian texts refer to a specific, evil being with super-human power. This being is named Satan. (Well, some Greek word that is rendered that way, I guess.) When translating texts using it as a proper name it would be misleading to render this as "the satan" because it's not referring to a role but a named individual.
The challenge, then, is how Christian editions of Tanakh (aka "old testament") should render הַשָּׂטָן. A scrupulous edition will render it as "the satan" or perhaps "the adversary", which is accurate, consistent with the original context, and non-doctrinal; Christian readers would understand it differently, but the translation shouldn't mislead. A translation that values doctrinal consistency among books would render it as "Satan" to reinforce the point; this of course obscures the original meaning. In the latter case a better edition would footnote the decision.
Please note: This answer was written for a neutral, academic audience and is not intended to be interpreted in the context of a religious belief or doctrine.