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English language certificate
If you want a formal certificate, than you can do an IELTS exam.
http://www.britishcouncil.be/exam/ielts
However, the last date they have for 2015 is currently 27th June. I suppose they restart in September?
Do you have a specific reason for wanting such a certificate or is it just for your own satisfaction?
If it is in connection with a job or further education it is better to find out exactly what is required and where to get it rather than spend time - and money - only to find that something else is needed.