Are Russian-English or Thai-English translators or interpreters in the highest demand?

Russian, I would guess. However, you have to also look at the supply side. More people know Russian, which means more competition and a saturated market. If you're a translator from one of the "rarer" languages, with some luck you can find a niche for yourself somewhere and face little or no competition. Also, the less common your language is as an interpreter, the easier it gets to work your way to prestigious jobs (embassies, high-level cultural exchange, etc.)