You must follow US tax law when reporting the sale and gain, regardless of how the gain might be reported in the other country. It sounds like your home country allows you to adjust the cost basis for inflation. The US does not.
Your cost basis is the fair market value on the date your father died, translated to US dollars at the exchange rate for that day. (Likewise your selling price is the US dollar value at the exchange rate for the day you sold.) You can include adjustments if you paid for permanent improvements, but you can't take an adjustment for inflation or market appreciation. If your mother co-owned the property, your US cost basis may be very different. I can't tell if she was a co-owner or if she simply handled the sale for you.
If this was a rental property (what were you doing with it these last 30 years) you have to decrease the cost basis by depreciation you claimed or could have claimed, over a time frame of 27.5 years, so you have no cost basis now regardless of what it was.
You will file a schedule D with your tax return to report and pay capital gains tax on the sale. It sounds like your cost basis is $8,000 and your selling price close to $140,000 so you do indeed owe some capital gains tax. You can assign any cost basis you reasonably believe to be true, but in the unlikely event you are audited, the IRS does not have to award any basis you can't prove.
If any form 3520 was needed, it would have been 30 years ago when you inherited the property. You don't owe the form now because you are selling property you own and any money coming to the US is your money, it is not a bequest or gift in 2021. And even if the property value in the 1990s was over the limit to file a form 3520, the statute of limitations is long past.
