Checks are not common outside of US and a few other countries. Checks from another country especially can be a hassle to cash, may take a long time (as in weeks!), and may require paying a percentage fee to the bank.

They're not terribly common even within the US. I refinanced my house recently, and and I had to order a new checkbook from my bank to do it, because I couldn't find the one they gave me four years ago. Every time I do have to write a check, I always have to "figure it out" again, which isn't exactly rocket science, but is a testament to how little I write checks.

Like most of the world, I use credit cards or debit cards or a cash sending app.

They're common enough for banks to universally support them. E.g. if you get compensation from some company (due to overpaid bill, legal settlement etc), it'll usually come as a check by default - and sometimes that's the only available option. I've also dealt with landlords and home repair providers that would only accept checks.

yet you can still cash a check and write a check even if its not popular.

Well, I mean, mailing a check takes a long time, certainly longer than sending some bitcoin would take, even if it was free. Emailing a scan could work, but wouldn't that be completely trivial to abuse? If someone gets a Gmail password, they could just photoshop a the check to empty out your relative's account with very low effort.

I think blockchain kind of solves these two particular problems; the money transfers are relatively quick (around 20 minutes for bitcoin), and you have all this cryptographic goodness to minimize the potential for fraud.

It shouldn't matter how long it takes to mail after the first check is mailed, right? If the relative in the US is sending money every biweekly pay check and it takes 9 days or whatever to mail a check, the relative in El Salvador is still getting a check every two weeks, just nine days later on the calendar than payday.

Lightning network transactions occur in seconds, if not milliseconds [1].

https://lightning.network/

You should let the El Salvadoreans know about all these ideas, might save them a lot of trouble

At least here in Argentina, if you have a check of a bank od USA, only a few banks accept it and they take a huge cut.