"I come from California."
Translation:Ego a California venio.
Are you sure the noun was "California"? There are nouns that can take a bare "ablative of place from which" without needing a preposition, but those nouns all have a distinct locative form.
"Romā veniō" -- "I come from Rome"
"Romae dormiō" -- "I sleep in Rome"
"A Californiā veniō" -- "I come from California"
"In Californiā dormiō" -- "I sleep in California"
California is not a city, town or small island. It's way too big to take a locative.
Besides the names of cities, towns and small islands, there are a few other nouns for which the locative survives into the Classical period, mostly cozy nouns such as domus (home), rūs (estate), and focus (hearth).
|domus||domū / -ō||domī|
I would think that in the context of place origins, “de” would be used rather than “a/ab.”
If I am coming home from vacation in California I’d say venio a California “I am coming from CA.” But if I’m away at college and telling my roommate where I come from, I’d say venio de California “I come from CA.”
Ultimately, what I’m trying to say is that if you use “a/ab,” I think a progressive translation would work better in English: “I am coming from California.”