Apple TV+ has no specific region. The Region is set by your Apple ID.
Apple TV+ Content is available in a number of languages, both for audio track and subtitles which can be changed at any time during playback from the menu in the playback controls in the Apple TV app.
If you change your Apple account to Mexico, you will need a Mexican credit card to make payments for subscriptions and purchases in Apple's platform.
If you are part of a family Sharing group, you will need to leave that Group before changing the Apple ID country.
If your daughter is part of the Family Sharing group, and keeps her account set to the US, then she will no longer be able to be part of Family Sharing with you, and cannot share the Apple TV+ subscription.
Family Sharing requires all members to have their account set to the same country.
The content in Apple TV+ will remain in its original language, that being English for most things in Apple TV+ and as mentioned you can change the language at any time. You won't be locked to Spanish if you do change though.
There is no specific Mexican version of Apple TV+. Apple TV+ is the same everywhere.
Changing your Apple Account process, is a fairly involved process, and you won't really gain anything by doing it.
click here ➜ Change your Apple Account country or region - Apple Support
Unless there's a substantially compelling reason to change the account to Mexico, I would not bother, it's just going to cause more trouble than it's worth. Keep it set to the US as long as you have a payment method from the US is the better option.
What exactly are you looking to accomplish by changing it to Mexico?
There is no hidden Mexican content for Apple TV+ if that's what you are looking for.
