How do you say something in Spanish with the subject "one"?

The best example I can think of in English is "One does not simply walk into Mordor." Yeah, I know, silly example. Whatever. Anyway, which conjugation would you use? Or rather, which conjugation would one use? Second person singular (tú) or third person singular (él/ella/usted)? Or is that simply not done in Spanish?

Jim2015-09-10T22:37:45Z

Unlike French, the personal "one" is not so frequently expressed in Spanish.

On parle français, On parle espagnol

habla francés, habla español

Or:

Se habla francés, Se habla español

One sees signs like that in shop windows to attract foreign tourists.

Mark2015-09-10T22:27:49Z

Usually what looks like a reflexive pronoun is used ("se habla español" means "one speaks Spanish", or "Spanish is spoken").