Is everyone obligated to follow the fatwas of his country, and if we adopt the fatwas of scholars of other countries, is it considered a sin?
All perfect praise be to Allah, The Lord of The Wolrds and may His Peace and Blessings be upon our Prophet Muhammad and upon all of his family and companions.
It is permissible for the person who isn't specialized in Sharia sciences to apply the opinions of renowned scholars whom he trusts ,whether they be from his own country or not ,but if the scholars have different opnions regarding a certain issue/matter, then he must consult someone who is more knowledgeable than himself .It is preferable that you (the questioner) specify the case of your interest ,so that we could give you a more specific answer since some scholars deliver fatwas based on illogical/atypical opinions which should not be applied no matter what .And Allah The Exalted Knows Best.
Is permissible to buy/sell with non-Muslim?
Yes buying/selling for a Muslim with non-Muslim is permissible as regards what is lawful.
What is the ruling on taking a loan from an interest-based bank (a usurious bank) to buy an apartment for housing, noting that I am renting at a high amount relative to my income, and I am an employee? Also, considering that the conditions of Islamic banks are strict, and I cannot obtain the full amount from them, is it permissible to take a loan to purchase an apartment or not? May Allah reward you.
Usury (interest) is prohibited by Islamic law, and the Messenger of Allah, peace be upon him, cursed the one who consumes usury, the one who pays it, its recorder, and its witnesses. Whoever leaves something for the sake of Allah, Allah will compensate them with something better and greater, and Allah will provide relief after hardship. Needing a place to live does not permit taking a loan with interest. And Allah Knows Best.
What is the ruling on making a false oath?
Making false oath indulges its maker in Hell; therefore, he is obliged to repent, ask Allah for forgiveness and make an expiation; feeding 10 poor or providing clothes for them and in case of incapability of doing any of latters he should fast for 3 consecutive days. And Allah Knows Best.