I find the Sapir-Whorf Hypothesis - the idea that thought is constrained by language - to be fascinating.

Here’s a variation on it from The Guardian

Take the Portuguese president of the European commission, Jose Manuel Barroso, for example. Barroso has an extraordinary knack: when he speaks English he not only talks but thinks like an Englishman; when he speaks French, he not only talks but thinks like a Frenchman. To hear him alternate from one to the other can be quite disconcerting, almost as if he’s switching between a left and right brain.