Say if someone owns 10 Bitcoins which is not declared and government doesn't know about it (earned/mined etc but did not buy) and now that someone want to avoid paying tax on cashing it out. Can he simply move to countries where there is 0 tax like (UAE) specifically dubai and setup a company there and cash out crypto over years in dubai bank account and later decide to move to his home country with all that money. Since dubai has 0 personal tax and his home country can't charge tax on it because of non-resident of home country. Is this possible?
First, when you hide your money and do not declare it, it becomes a black money for you and it will hard to cash out (if it is in large amount) as the authorities will as for the source of that money.
As far as if he goes in Dubai (or any country which has no tax on bitcoin), cashes out, develop a business etc, that is ok but when he will move back to his won country he would have to pay his taxes on the amount which he currently holds as per his country law. There is no way you can escape from tax.
Secondly. is there no tax on bitcoin in Dubai and when you convert that bitcoin into fiat, won't the Dubai government charge tax or is there no tax on fiat money too
That's strange by the way.If you are planning to legalize bitcoins through a business in Dubai, then first study the costs of starting a business in Dubai, the cost of licenses and additional costs of living in this country.