You shouldn't have any problem logging in to your Facebook account, regardless of what language has ended up set as your preference: even if you're working in a different character set or system language, it shouldn't adversely affect your account name or password. If that's happening, then I'd email Facebook support and see if you can sort it out.

Assuming that's not the case, it's tricky but doable for you to reset your default Facebook system language. I'll show you how to reset it from Spanish back to English, but the key idea is that you need to know where things are located on the screen rather than worry about the words. That way, if you're in a language that you totally don't understand, you'll still be able to succeed.

To make double sure we're successful, let's map out the journey from being logged in on Facebook to being able to see the language settings in English first.

From your home screen, you want to click on "Account" on the top right:

Count things out: the entry we want is the third one down. In English it's "Account Settings":

Now there are a set of language options along the top. We want "Language", which is the fifth tab, as you can see. Click on it and you'll see a menu that displays your current language. Here, of course, it's "English (US)":

To change it to something else, simply click on the menu and pick the language you'd prefer...

I'll choose "Spanish" and go back to the home screen to see what Facebook looks like en español. Looks, uh, like it's in Spanish. :-)

Now, to reset it without knowing the words, we can simply count entry slots. On the home page we still want to pull up the menu on the top right and choose the third entry. In this case, however, it's the "Cuenta" menu and the setting we want is "Configuración de la cuenta":

Count along the tabs to find the fifth entry, here labeled "Idioma":

and we're back at that pop-up menu of language choices:

Simply pick "English (US)" again (or whatever language you'd prefer) and the page instantly changes into that language and resets everything on your account:

Es muy bien! Now, just for fun, here's what the account menu looks like in Croatian (Hrvatski), Japanese and Russian:

Yeah, I switched it back to English when I was done with those screen captures. :-)