May I ask what the purpose of the second language is?
Depending on the answer you wan to consider not just language but also country and culture.
If you are selling things, a second language is not all you need. You also need a local address in countries you are selling, different currency, potentially different payment methods, etc.
If you are also advising or talking you might need different policies (min. age, corporate tax ID, etc.) to be compliant.
And in any case you need to take local dialect into account as well as culture and customs.
I think local domains are favored by local search engines, like Google Mexico. So if you want to really have an impact in a different country it might be worth exploring not just a second language, but rather a local presence.
If it is only the second language, I'd favor a
negotiated content approach, because you can fall back to the first language version with an IFrame that indicates that the page is not available in the second language. You can also piece by piece replace cultural sensitive content and even differentiate between spanish Spanish and mexican Spanish. You can also relatively easy determine if a particular page has been changed in the first language and needs updating in the second.
K<o>