Hello, Scott.
The apostille is an international convention that is about a century old. It is how most countries recognize documents issued by other countries. (In other words, this is not a ******-vs-Mexican question, as many have suggested elsewhere: the apostille was established in The Hague ages ago as a way to simplify international paperwork.)
One may aposille only official documents. To do so, one presents oneself to the local issuing authority and requests a certified copy of the document (in your case, the marriage certificate from the county recorder in Seattle). The document is then taken to the state agency in charge of apostilles (in California, that is the Secretary of State; I don't know what it's called in Washington state), where the document is given its apostille. This is nothing more than an international certification of the local certification of the original document. As such, the document originally issued by the sovereign state of Washington ought to enjoy full faith and credit before any of the nations registered with the Hague Convention of 1920-something-or-other.
With the apostille you have a document that is accepted internationally. All you need at this point is to render it into one of the official languages of the receiving country.
Although there are about sixty languages being spoken in Mexico, the only official one is Spanish. And how might you translate your documents? Well, by paying an English-to-Spanish translator, of course. But no ordinary bilingual will do: you must use one who has been certified by your governing body. You need to look for a "perito traductor" -- this is someone who has been appointed by one of the thirty-one states or by the federal district of Mexico to offer formal translations for the purposes of good government etc etc. (Please don't ask me: I am merely a literary translator.)
If your agency in Seattle is authorized to translate for any of the federated bodies of Mexico, then you have no worries. If not, I would suggest you find someone who is authorized to translate for the Instituto Nacional de Migración and have them handle your paperwork. Best to get their guarantee in writing. And aim for an FM1, not an FM3. Because of your wife, you should be able to achieve dual citizenship in a couple of years.
I write from Tijuana, where everything is a bit of a blur. Here we are all fronterizos and much is forgiven on that account. (I spent two years bringing carloads of household goods into Tijuana that were not on our menaje de casa.) You will not enjoy the same latitude by moving from Seattle to Jalisco. To make matters worse, I doubt that your house will fit in one van -- ours didn't and we came from a two-bedroom apartment. Before you go, find a place where you can store "stuff" without payment or compromise and close to where you are likely to drive up to visit.
If I had the luxury of going back in time to give myself advice, it would be to say "Don't worry, just prepare." And, if I were in your position, I'd get my FM1 now from the Seattle consulate rather than later from INM. Yes, an FM1, not an FM3: being married to a Mexican puts you on the fast track to dual citizenship. Use it!
Your move appears to be daunting but it is merely a question of putting one foot in front of the other. Well, for me it was -- I was already acculturated before I got here and I suggest you do the same. I've seen culture shock and it isn't pretty. Being married to a Mexican is not enough, one needs to accept Mexico as an alternative and proper reality.
Mexico is an alternative and proper reality, carajo!
