So my mother tongue is not English

Your written English is very good indeed, there's no doubting this.

Simply put yourself in your prospects shoes. Ask yourself...

What is their #1 frustration or pain right now?

What is keeping them up at night?

What emotions are they experiencing?

And then give them exactly what they want to hear.

Keep your language very simple and use a mixture of longer and shorter sentences to keep their attention.

The purpose of the preheader (if writing long form sales copy) is to get the reader to read the main headline.

And the purpose of the main headline as well as containing an irresistible benefit (hook) to grab their attention... is to get the reader to read the first sentence.

And the purpose of the first very short sentence is to get the reader to read the second sentence. Etc etc...

Every word you write, every sentence constructed will affect your readers emotions. Try not to jar on those emotions.

In a nutshell create a slippery sales funnel so the reader slides all the way down to your direct call to action. And make it very easy for the prospect to take your or your clients desired call to action.

I wrote some copy for an MTurk requester - he paid out, but didn't like it, and sent me the comments of his reviewers.

One assessor questioned whether I am a native speaker of English (the only subject in which I came top of the class in my Premier League English public school). Another had elementary spelling errors in his comments. The third had some valid comments about the "number of facts" in my submission, but was clearly unable to distinguish between qualitative facts and quantitative facts. Dang Yankees.

If you have already put the stuff into English, you should be able to find someone on Fiverr to "straighten it out" fairly cheaply. If the man you're working for rejects it, try running it past a different "native" next time.

Hey, you must be pretty close, or you wouldn't have got the job in the first place.

Ken - that's the first time I've seen anyone insist on Oxford punctuation in Illinois. Are you originally from Boston?

I'm struggling to comprehend what just happened..
It would seem that a representative from Asia, the worlds leader in crap article production has laid the challenge.

The biggest problem with us non-native English speakers trying to write copy is wordiness (especially if you pride yourself as having good English skills). Second biggest problem is the inability to get into the head of the prospect and speak the way they think.

A good way around this is to make your character foreign and play off the stereotype. This way you're allowed to make grammatical mistakes, to not be exact with your language and even use your own voice with an accent in video and audio.

Here is a very crappy example just of the top of my head:

"Hi, My name is Amit, I'm originally from India, and everybody knows that we indians are very good programmers. Well let me tell you a secret beyond secrets. We're not really better at programming, than all the other nation, we're just very good at selling our programming services to Americans. And now, I'm going to show you exactly how you can make millions selling programming skills to small and big businesses in America and worldwide..."

As for which English is correct or not - English that sells is correct as far as copy is concerned, English that doesn't sell isn't. Everything else is just pomp.

Here is what John Carlton answered to a very similiar question in one of the comments to his blog post:

Yes, I know several writers for whom English was a distant second language… and who had trouble expressing themselves well in it… nevertheless succeed at selling with copy. However, they mostly worked for themselves — and what they did (and what I advised them to do, if they weren’t already) was to just “own” their not-great English… and become The Crazy Hungarian, or the Ninja Marketer, or the Russian Fool… whatever handle they felt comfortable with. Most English-speaking consumers go easy on folks who don’t speak English so well… especially if there is some humor added to the attempt, or some mild self-deprecation. The main thing: It’s just an obstacle… and obstacles are just impediments to fast progress that you must overcome. You figure out how to position yourself either with the obstacle in tow, or find a way around it… and keep moving.

OK, this is one of those threads I have read in full and slow pace. Thank you for your comments. I guess it's just a matter of practising.

Pete Walker's first comment was great.

Pay someone to copyedit your translations.

For instance, in this case, "need some texts here and there" is an awkward construction and wouldn't be used in standard American English. "I am in front of a language barrier" also is a phrasing that wouldn't be used in English... and so is "Being (name of language) my mother tongue."

You've got a comma where you don't need a comma (etc.)

I don't mean to sound dismissive, here, but you really do need to hire a good editor for your copy. Your English isn't "academically correct." (I have sold articles to textbook publishers, so I'm pretty sure I know what "academically correct English" looks like.)