i dont think theres any language for 'thought'.
no matter what language u were taught to express yourself in, ideas are all the same.
for example. say telepathy was real, and we can think something and somebody can understand it, without needing to physically express it. does that mean we have to know what language they know to telepathically understand them? i dont think so, thinking involves no language. so if a japanese telepathic man trys and reads the mind of an english telepathic man, theres no hold ups, the japanese man understands even tho the japanese man doesnt know english. same with the english man comunicating telepathically with the japanese man.
so no, i dont think anybody 'thinks' in a certain language. when trying to express the idea however, thats fully up to what language you were taught.