When publishing an app in a store, such as Apple's App Store, you choose in which countries the app is available. See the instructions on how to do that for the Google Play store, for example.
Smartphone users have to set their base/home country in their smartphones during initial setup (they are able to change it later). They can only download apps that are available for their configured country.
That said, if there is ever a dispute, it will usually be settled according to the laws of the user's country.
If you do not make your app available outside Mexico, you should care mostly about Mexico's laws.
EDIT: Do notice though, though, that laws from the country where you are hosting or processing data will also apply. For example, if you would host in Europe, due to the General Dat Protection Regulation (GDPR), you should have to allow users to remove their data from your system if they so wish, even if they are not physically located within EU borders.
It might be worth the time to consult a lawyer to check a few more things. For example, if your app is going to have users that move between Mexico and the US frequently, then you might consider the Children's Online Privacy Protection Act and in the very least not allow people under 13 to have accounts in your system.
To really simplify things, you could find a company that would host your app in Mexico. That would make ypur life easier regarding international laws.
I am not a lawyer, so as always, talking to an actual lawyer might give you more clarity on these things. In the very least you will be more confident about what you can and cannot do.