If a stranger from different country visited another one, which law will be applied on him?

The law of whatever country you are in applies to you. Depending on where you travel from some laws from your country of residence may apply to you while you are in other countries - this is not usual though. For example, if a person from Australia goes to a SE Asian country for the purpose of paying for sex with an underage sex worker there may be grounds for them to be prosecuted in Australia.