Is "Mr./Mrs. + First name" ever okay?
Is it okay to just use Mr./Mrs+ first name instead of last name in countries where last names are very long, and people don't call each other by last name?

For example, a Thai person's name is Sombat Tanakritcharoenwisut.

I want to compare the name with an American, let's say John Smith.

People would call him either "Mr. Smith" or "John".

To put it in the same pattern, it would be either "Mr. Tanakritcharoenwisut" or "Sombat". But it won't be practical in real formal conversation because no one will be able to pronounce it or even remember this long last name.

Is "Mr. Sombat" okay just because it's easier to say?
Thank you.

Question

Is "Mr./Mrs. + First name" ever okay?
Is it okay to just use Mr./Mrs+ first name instead of last name in countries where last names are very long, and people don't call each other by last name?

For example, a Thai person's name is Sombat Tanakritcharoenwisut.

I want to compare the name with an American, let's say John Smith.

People would call him either "Mr. Smith" or "John".

To put it in the same pattern, it would be either "Mr. Tanakritcharoenwisut" or "Sombat". But it won't be practical in real formal conversation because no one will be able to pronounce it or even remember this long last name.

Is "Mr. Sombat" okay just because it's easier to say? 
Thank you.

Answer

It depends on the country and, more importantly, on the person. In the country where I live, my students call me Mr. Jason, but back home it would be Mr. Lastname, though I remember attending a daycare as a kid in the US where we would call the teachers by Mr./Ms. and their first name. Also, I had several teachers over the years who would tell us to just call them Mr. S, for example, because their full last name was difficult to pronounce. That's not uncommon in a class setting.

It's perfectly acceptable to ask how they prefer to be addressed and/or how to pronounce their name. This is the considerate way to handle the issue in any situation or setting.