First please get some facts right! El-Salvador did not just make bitcoin legal tender but they made it the official currency. Mexico is not going to do the same! They are going to adopt bitcoin as a legal tender alongside their official currency system. So both cases are different.
That's a wise way to do it unlike El-Salvador! This way Mexico will be able to protect themselves from bitcoin's volatility while enjoying the innovation!
What is actually the difference between a legal tender and an official currency? I think they're the same. In El Salvador, it is not just Bitcoin that is the official currency. The US dollar remains as one of the country's official currency alongside Bitcoin. El Salvadoran businesses cannot abandon the US dollar as a payment option because it is also the country's official currency. They cannot refuse customers who prefer to use the US dollar in buying goods and services.
I think this proposal to declare Bitcoin as a legal tender in Mexico is still far from happening.