how will I declare this to my country the day i cash out ?
1. In what country you are living?
2. If there's no regulation about cryptocurrency on your country, you can declare it as it is - a cryptocurrency - a digital money that has real fiat value.
Will they try to be interested in where this money comes from exactly ?
If you will cash it out through bank channels and then the amount is high then the bank might be interested and will question you but if that will be sort of remittance, I don't think they will be interested to your money.
Or will they just accept that someone send tokens to me just like that ?
You need to research about your countries status about crypto.
The laws about bitcoin taxes are blurry, but the law about tokens are even more blurry
It still depends on where you live.