I think it would be dead simple. I used to enjoy cracking puzzle codes.
There are only 2 single letter words, a and I. A handful of 2 letter words. Why would they be anything except simple conjunctions, etc? 26 letters with very well defined structure (vowel in every word, only a few used in pairs, easily determined distribution, etc). 3 things that end sentences (.!?) Capitalized letters. No diacritical marks.
Take me a couple of hours.
Dennis
Easy to put to the test: try translating a sample of, say, non-Romanized Pali. No cheating by cracking open a dictionary!
You see, you guys are approaching this all wrong. English is not a code. It’s not designed to be difficult. It’s designed to make sense and follow structure. Now granted you wouldn’t know that but it drops out very quickly.
Letter, word, sentence, paragraph. No columns or anything else odd. Punctuation marks are few. 99% of marks within a sentence are commas. Why would they be anything else but to break up the flow, to pause, to slightly separate? 98% of ’ are followed by an “s”. There are only so many things that can mean.
My Rosetta stone example is a Sriracha Sauce bottle. Take a look.There are 5 languages on it. English has the fewest words, 14. Chinese uses 60!!
There are only 5 vowels and there is at least one in every word. That’s .0000001 seconds of CPU processing time to indicate they are vowel sounds. People in the future don’t just magically have the ability to speak only in consonants. OK, forhet that “clicking” language 
I don’t know about non-Romanized anything. I assume it is a structurally difficult language. But I repeat, English is not.
Dennis
Well why don’t you go ahead and knock out Proto-Elamite this weekend? Might be a Nobel in it for you.
I agree with everything you said (except, perhaps, Pali being “structurally difficult”, though I grant it is highly inflected-- the instance was supposed to be of a language that was still Indo-European but written in an unfamiliar script for you to decode). No natural language is supposed to be a code; that does not hold for English only. I just think it should take more than two hours to learn and understand a language to the point where you can translate it. Even Michael Ventris took a couple of weeks.
(Try Indonesian, by the way. One of the most widely spoken languages on Earth, you already know the writing system, no funny inflections or genders to confuse you…)
I did that last weekend. Give me something more challenging!!
Anyway, as I said in my first post and as others have said, the ease of translated English would depend on the availability of languages like French and German. If you had both of them at your disposal (and maybe Latin), it would be a piece of cake. If you didn’t, and you didn’t have any Indo-European languages, it would depend on the texts you did have. If it was a few scraps here and there without illustrations, you might as well forget it. If you had the Encyclopedia Britannica, you’re back near piece of cake territory.
It is very easy to explain the rules of English having already known English, like the Texas Sharpshooter. But how about a language that you don’t already know? For instance, Korean? (Assuming that you don’t already know Korean.) Korean is not a code. It’s not designed to be difficult. It’s designed to make sense and follow structure. And it is alphabetic. So here is a Korean site that (I believe) is for self-publishing ebooks on-line. So vast amounts of text to work with–so my challenge to you is that you go to that site and with no cheating (which includes any and all web searches for information on the Korean alphabet and translation engines) I want you to derive the structural rules of Korean and translate some of the text into English. I can’t say if Korean is more complicated than English, so I’ll be generous and give you twice the time you would need for English–you have four full hours to decipher Korean. Should be a snap.
While the size of the Proto-Elamite corpus is fairly decent, it’s not an alphabetic script. The number part has been figured out but the symbols for the objects being counted are still a mystery. I.e., “Is that a symbol for a cow or a loaf of bread?”* That’s really hard to figure out.
Alphabetic scripts like English are just a whole lot easier.