I always assumed there was no thai word for billion, because I always heard it as "a thousand million".

How would one say "1 trillion"?

Lahn lahn ล้านล้าน

Maybe this is off topic, but I find it somewhat odd to say there's "no word" for billion, which might be taken to imply the idea can't be expressed. Of course, by this same standard English has "no word" for ten thousand (Thai has หมื่น), or hundred thousand (Thai has แสน).

Neither language is hindered in the communication of these concepts, though. One word or two words -- don't see the difference.