Bad news. Proof-checking is not that easy. Really.
Over the many years in Poland I have also turned my hand to proof-checking a few texts, all of them from texts that were originally translated from Polish to English. Many of them are reasonably straightforward, but I have come across many that frankly Google translator could do better. I think the reality is that the translator did indeed take the lazy route and just ran the text through an online translator or application.
But let's say that you decide that your lovely translated piece of text into English should be to a standard that you feel is one that would be properly written as if it had been penned by a native speaker. Once I receive the text, particularly if it is from someone I have not worked with before, I look over it to assess whether it is indeed proofable (especially if, as I've mentioned above, it has been put through an online translator).
The next task is then to not only make the corrections, but also to correct and even rewrite texts. All this is necessary to ensure that the text is as close to the original in style, prose and spirit. It also must give the impression that it was, in fact, written by a native speaker.
One has to hand many dictionaries, thesauruses, even access to encyclopedias, particularly if you are dealing with factual texts (one has to check that the given translations of names and events would be the same as those given in any established English factual books). And then when you've done that, you have to go through the whole thing again. You would be surprised what you missed first time around. One tool that I find invaluable - and which can be found on the 365 version of Word - is a reader that is able to read back your texts aloud. (I use British English Balabolka.) It's amazing how much you miss if you don't use this, or even get somebody to actually read it back to you.
The biggest danger is rewriting. One of the most frequent problems I get with texts for proofing is the writing style. Often many texts I get for proofing are written to a certain style, which can be 1) The local language academic style or 2) an overly formal style. As an example, I frequently get texts full of short sentences because 'that's an example of good writing'.
Not if it reads unnaturally, it doesn't.
Good writing means what helps you to express your message in a way that communicates both clearly and persuasively, particularly to your particular audience. Register is very important. Academic writing is dull and depressing and not enjoyable to read. If I am presented a text that is overly formal in register, then I will either not take it on or rewrite it (and charge more) so it retains its formality but becomes enjoyable to read. And if your audience places that criteria above a set of narrow-minded grammar and register rules, you've won over half your battle.
But also one thing that can happen is that if a particular phrase is ambiguous, then one can unintentionally change the grammar or vocabulary that inadvertently changes the meaning. If it clearly makes no sense at all, I will simply say so in the comments box.
I may even add a few additional ingredients of my own in order to 'spice up' the texts. In some cases I have done this, but I do inform the author of what's been done as they have to agree to what is extra text that did not originate from them. In one of the books that I worked on, the journalist wrote a series of texts that were generally written in an informal and semi-humorous style, and there was one translation that provided the gift of some wordplay jokes which would not exist in the original language. I added these jokes and these were accepted.
Sometimes cultural references need to be changed: what is perfectly clear in the original language may leave English readers completely lost. Where possible, I'll try to find an English-speaking equivalent. There was one I was rather proud of, but in the end it was edited out for reasons of space: the author quoted lines from a popular Polish song about the joy of singing. English readers would have no idea of this song, so I chose something that English readers would understand, but keeping to the spirit of what the text was trying to say. (It was The Carpenters' 'Sing-a-Song'.) Anyway, those books have been published and you can find out a little more here.
Now translations I would stay away from are stories. But I will break my rules when it comes to fun poems. For example, there is a popular Polish children's poem called Okulary (The glasses), but the 'official' (there is no official, but there is one translation that seems to be accepted as gospel. Check Wikipedia) is, frankly, appalling. I went as far as even translating myself and then proofing it. I do have an ambition to translate all of popular Polish children's poet Julian Tuwim's rhymes - not only the challenge of rewriting but keeping to the timing, pace and spirit of the original. It would be a challenge I would relish.
Finally, if you need some texts proofing that are written in English, then do contact me (click here). But I draw the line at overly technical texts, specialised texts in specialist fields, or texts that look like they have been put through an online translator. I believe my prices are reasonable!