why farang/foreigners use chopsticks with Thai food

In short, if you're eating from a bowl you can use chopsticks, if you're eating from a plate, use fork and spoon. Just my very subjective opinion of course ;)

Kananga's great eating directive 2010

Where there are multiple utensils containing a fork such as a knife and fork or a fork and spoon the fork must be placed in the left hand. Never use the knife to deliver food to the mouth or to come in direct contact with the oral area.
The utensil used to bring the food to the mouth is always the fork unless there is a spoon present where it would superscede the fork. The fork must still be placed in the left hand, however it must not come in contact with the oral area when the other utensil is a spoon.
If there are chopsticks and a spoon the spoon must be held in the left hand, although both the spoon and the chopsticks may be used to deliver food to the mouth.
Do not underany circumstance attempt to use a spork in public. With any hand.

You know what really gets my goat is Westerners (mainly UK) who seem to think you should hold you knife like a pen.

Fart's eating directive #1:

You should always hold your knife like a pen.

Actually... and I'm surprised it's taken me this long to ask this question...
When you get a fork and spoon in thailand, which hand are you meant to hold each of them in and which one is meant to go in your mouth?

Is Kananga right or is he just having us on??

What if someone ifs left-handed?

The spoon is supposed to go in your mouth but I see people use the fork all the time. Which hand you use is of no importance.

left-handers such as myself will usually use their left hand for the spoon. When they eat with just a fork, they will also usually use the left hand. Same goes for chopsticks.

because left handers are used to living in a world that is designed for the 90% of people that happen to be right-handed, they often have greater dexterity with their 'other' hand than right-handers do, so sometimes they might eat with sticks in their right hand rather than their left. This doesn't make them either right handed or ambidextrous though. True amibdexterity, when somebody is equally proficient with either hand, is exceptionally rare though quite a lot of left-handers suffer from the delusion they are ambidextrous when they are just better at using their 'other' hand than nearly all right-handers are.

It is a scientific fact that left handed people live on average 8 years less than right handed people.

You might find that left-handers are way over 90% - I've heard as high as 40 % - one problem is that a lot of left-handers don't know they are left -handed.
In many cultures doing things left-handed is regarded as suspicious or evil - so I wouldn't b at all surprised if you get some raised eye-brows especially amongst older folk.