A few years ago, I drove a 2007 Ford Focus station wagon. My wife drove a 2008 Ford Focus sedan.
Collectively, would you say we owned Foci or Focuses?
Focuses.
The International Trademark Association recommends never using a trademarked name in the plural or possessive, unless they’re already in that form. Thus, they’d want us to say something like “two Ford Focus automobiles.” The ITA however has no police powers over usage of the English language. In fact, nobody does. You can say what you want and nobody will arrest you.
I say Ford Foci and Toyota Matrices because I’m a bit of a jerk–not because I think it’s correct.
How do you pluralize Kia Spectra or Mazda Millenia?
I would probably do the same, but don’t think I’m being a jerk by doing so. I just like to play with language, especially alternative plurals.
Spectrae and Milleniae.
Who’s going to be the first to tell me those aren’t correct Latin plurals?
Actually, the correct plural would be “spectrata”
<ducking and running>
Neither. Smoothness is important to me in language. I would say, “We have two Ford Focus vehicles in our family.” I try to avoid what I feel are awkward expressions even if they aren’t syntactically incorrect per se.
As noted there is no legislation on this, but I pluralize proper names by adding s or es. A Focus is not a focus. Ford Focuses. Another example: Toronto Maple Leafs (common usage, not my personal preference).
I don’t think Latin is your first language.
Well, of course not. I started with classical Etruscan.
There is a fairly common last name of “Foote”. Of course people are going to want to pluralize it as “Feete” 
Any models called Charisma? I’ve heard the plural of that is charismata. Although how one has more than one charisma puzzles me.
“What type of car do you own?”
“Ford Focus. In fact, I own two of them.”
“I park them in the ellipse.” 
You’re thinking of charisma as the specific one which we can also call “charm”. But originally, charisma means “gift as in talent”: so, someone who’s a gifted singer and good with languages has two separate charismas (I’m not going to use the Greek plural when speaking English :p); note that I haven’t touched on whether he is or is not in possession of the kind of charisma we speak of when we’re talking of politicians or actors.
The second half of that sentence is hard to reconcile with the first half.
Did you lose focus?
I might say “Foci” or “Tauri” or “Prii” (actually Toyota’s preferred Prius plural in English) once as a “joke,” like “My dad owned a couple of Tauri before switching to minivans when he got grandchildren.” (And now I can never say that again, regardless of my audience, because it’ll sound horribly forced to me instead of vaguely clever. You’re welcome, family. My apologies, fellow Dopers.)
But as a rule, I’d say “Focuses” or “Tauruses” or “Priuses.”
In my opinion, trying to use Latin or Greek rules today is pointless and confusing.
I agree with CookingWithGas who said “I pluralize proper names by adding s or es.”
Given that television is a mixture of Latin and Greek, what plural would classicists use?
We** can** end a sentence with a preposition.
It’s not Latin or Greek, but we can also use a split infinitive too!
Amen.