Firstly you need to consider your target market - who are the people you want to 'talk' to?
This should influence the tone i.e. you approach a professional looking to buy a product for their business in a totally different way to a mother buying an xmas pressie for their child.
Once you've worked out who you're talking to, you need to work out what they will be looking for from the product or service you're selling them i.e. someone buying for business may be less bothered about price but more bothered about delivery and quality.
Work out that the benefits of your product/service are and focus on those. Try to tell people why they need the product, instead of how great it is etc.
There's loads of advice out there but remember it needs to be compelling, straightforward and easy to understand.
Depending on what exactly you are writing. E.g. web copy would be different from a magazine article. Keep that in mind. Also, make sure you have an introduction, main story and a conclusion in your article, so it reads smoothly.
I think that my target market is very wide! I want to start copywriting in English. I have some difficults with open up world of English business language.
anglicising an article is quite easy; it comes natural to know what and how to say it if English is your first language of course, so editing is easier I agree. It depends how poor the written and understood English really is though.
The OP would have to give us a sample paragraph and we could rate them out of 10 whch might indicate how far they have to go. Or like you say, sell em cheaper because they need anglicising but the price reflects you having to do this bit yourself.