I have been approached by someone to translate a foreign language into (written) English. What is an acceptable rate (preferably in UK currency, but US would be fine) per thousand words that I could charge for such a service?
TL;DR - It's a balancing act between their budget and your experience. Take their budget, divide by time, adjust down for lack of experience if you're not the only horse in the race = rate. /end tl;dr
First up a disclaimer: I am not a translator. The advice I have to give you is based on my experience as a freelance illustrator and artist. However, I believe it is applicable due to the similarities I'll cover later.
The first thing you should establish is what your client's budget for the translation is. That's not to say that you gouge them for everything you can, of course, but knowing what they're willing to spend does two things: a) you don't overprice yourself into giving the work to someone else and b) you don't underprice yourself so far that you're losing money.
Secondly, you need to remember that it's not just an hourly calculation of your time that you're charging for. You're also charging for however many years of learning and experience you've had with the language you're translating.
These two aspects considered, you should be able to parlay a rate that you and your client are happy with.
example (totally off the top of my head): Client has a budget of £4000 for translation overall. The script is 20,000 words. Seems like an easy division of £200 per thousand words, but let's just say you've only recently become fluent in the origin language (assuming English is your mother tongue). If you know the client's budget, and you know they are approaching other, perhaps more experienced, translators, charging the full rate might be stretching it some.
There again, if you're 20 years fluent and familiar with the language (and can demonstrate things like local anachronisms rather than straight transliteration), you could fully justify and maybe push for more than the simple £200 per thousand.
You can query the client's budget with simply asking how much they're willing to pay you. Try approaching it in a more roundabout way, such as demonstrating that you want to show them you're a good deal or if you're being exclusively approached (i.e. the only runner) whether it's "worth your time".
As I said in the tl;dr, it really is a balancing act of accommodation to each other's means. I believe it's as applicable to translation of films as it is to art because it should be more than straight Google Translation you're offering, and as such you'll be executing a kind of art in itself, giving English language nuances to foreign phrases for example.
sources: my own experience as a freelancer.