I have some bitcoins that I've kept for a couple of years and I'd like to sell them for a profit, without paying a big tax on it, and then to transfer the money through bank transfer to my country. Here in my country I'd have to pay 20%, which is not acceptable.
you need to establish tax jurisdiction first---often a timely and difficult process, and one which usually requires that you establish residency. in malta for instance, that takes physical presence on the island for more than 183 days/year and also the general intent to reside there. they don't want people renting an empty house while living abroad and using malta solely as a tax shelter.
establishing residency in a second country for tax purposes also does not automatically remove your tax liability in your home country. that depends on the tax laws where you currently live and/or do business.