Cambodians primarily speak Khmer, which unlike most languages in the region is not tonal, but makes up for it with a large assortment of consonant and vowel clusters. Young Cambodians prefer to learn English as a second language and you will find people who speak anything from basic to fluent English in major towns and cities. In tourist market situations, most Cambodians will know enough English to complete a basic transaction, though many vendors carry calculators into which they punch numbers and show you the screen to indicate the price.
A few educated senior citizens can also speak French, a relic of the colonial period when it was a medium of instruction in schools. Because the Khmer Rouge targeted for extermination anyone capable of speaking a foreign language, actually encountering anyone fluent in French is very rare outside Phnom Penh. German and other European tongues can be found in the tourist centres (but are even rarer than French) and Japanese is also a popular language for tourist industry workers. Nevertheless, if you cannot speak Khmer, English remains by far your best bet.
Chinese dialects, Thai and Vietnamese are spoken in Phnom Penh. Thai is more prevalent in northwestern provinces, whereas Vietnamese dominates southeastern provinces.