Fractal is the way to go here. Try this on for size, some trimming may be needed for it to fit perfectly. I'm going to be assuming she speaks Latin fluently, and the rest of the party speaks modern English where assumptions need to be made.
High Concept: Language Barrier
Aspects: Romance Language Family, Ancient Tongue, Seeming Similarity
Approaches: Complex +1, Local +2, Idioms +2
Behavior- Every time an attempt is made to communicate, the GM may have Language Barrier start a conflict. The PC is attempting to force a concession from it, treating it as a normal obstacle. Language Barrier tends to maneuver, attempting to place aspects representing miscommunications on those trying to communicate. ("No thank you, I eat bees" "Help! I hate you!") It will either use those aspects to invoke for effect (namely, an ill advised action) or use them to attack the PCs stress track, trying to get consequences related to the language barrier. ("Aqua means Fire" "She gratiasing hates me.")
Important note- Assuming the PC has previously caused a consequence on Language Barrier, they can (and probably should) invoke that consequence to communicate unimpeded- as long as it's worth the FATE points, and the consequence is still around. (Language barrier heals consequences as normal.) Another important note- Language Barrier should be an aspect of the inhibited character, allowing for compels to create automatic failures where GM feels appropriate. GM may always not have Language Barrier attack, should they decide.
Why not use a consequence or an aspect?
(IHMO applies to everything following this. These are both possible ways to do it, but I think a fractal (though possibly not this particular fractal) is the best solution.)
Aspects are generally something that works both ways. A good aspect helps you as often as it hurts, and if you want this to be a disadvantage, then this works better. That said, an aspect is not a bad fit- certainly there's no problem with having one aspect that's mostly bad, and with creativity this could be used well. I used this longer way because "use an aspect" is a three word answer, and I assumed the OP was aware of that option. Also, there's little a character can mechanically do to combat an aspect. (Other than change it at the next opportunity, and narrativly fight against it.) Consequences on the other hand are uniformly bad. (Not that you can't invoke a consequence, but that having a consequence = bad thing in the mechanics of the game. It's something to avoid whenever you can.) Starting play with a consequence is equivalent to starting most games with a serious hit point loss- possible, and an option if you don't mind a mechanical penalty for coming up with an interesting story. As a GM and as a player, I tend to prefer rewards going to players who come up with cool stories.
The advantage of the fractal is that you can damage it. Over time, you'll inflict consequences on it, which let you bypass it easier, both by invoking it's consequences and by the smaller amount of stress it can take before conceding. These consequences are sections of the language where you figure out how it works. (Say, certain words you know know the meaning of, or parts of the grammatical structure.) Eventually, you'll take it out- not just force a concession, but a complete victory. At that point, you can make it go away for good, just like if you took out a bad guy, you could kill him or otherwise completely remove them from any future encounters.